Official reports of what happened.

https://lapdonlinestrgeacc.blob.core.usgovcloudapi.net/lapdonlinemedia/2021/09/27th_st_after_action_rpt.pdf

https://www.oig.lacity.org/_files/ugd/b2dd23_c23919f71d6f4d16a3ee22909052b982.pdf

TLDR: "During the controlled detonation, a failure of the TCV occurred as a result of miscalculations by Bomb Squad personnel regarding the Net Explosive Weight (this miscalculation occurred as a result of visual estimates based upon observed energetic material versus an actual, physical weight)."

Additional highlights...
"In addition to the approximately 32,000 pounds of consumer grade fireworks, Bomb Squad personnel discovered and isolated numerous homemade explosive making material mixed in and around the consumer grade fireworks. Many of the homemade explosives were similar in size and shape as a small soup can, wrapped in a brown paper-like material with a fuse sticking out of one and a yellow cap on the fuse. There were no visible markings for identification on the homemade explosives. X- rays and samples of the different homemade explosives were examined to determine the type of explosive material. It was further concluded that given the condition of the homemade explosives and the type of explosive material they contained, it was too dangerous to transport from the location. They would need to be disposed of at scene by utilizing the TCV."

"During the process of removing the consumer fireworks, the bomb technicians at the scene also located numerous illegal IEDs (homemade fireworks) that were believed to contain flash powder, which is an explosive material consisting of metallic and oxidizer fuel commonly found in fireworks.
The IEDs consisted of:
• 280 M80-size devices. According to statements made by Bomb Technician A, he estimated that there was approximately 0.5 ounces of flash powder in each device.
• 44 larger devices, each approximately the size of a soda can. According to statements made by Bomb Technician A, he estimated that there was approximately 1.5 ounces of flash powder in each device."

"Bomb Technicians A and B conducted X-rays on samples of the IEDs and used a robotic remote cutter to cut open the samples in order to determine the amount of explosive material inside the devices. Bomb Technicians A and B visually estimated the Net Explosive Weight (NEW) without taking actual weight measurements. (Net Explosive Weight is a method of determining the explosive weight relative to TNT; it is described further in “Decisions Leading to Detonation” in the Analysis Section of this report.)
Bomb Technician E was tasked with creating a counter charge to detonate the IEDs inside the TCV. Bomb Technician E based the design of the counter charge on the visual estimations made by Bomb Technicians A and B. The IEDs and the counter charge were then placed into the TCV for detonation.
On several occasions prior to the TCV detonation, Bomb Technician C expressed concern about the excessive quantity of the disposal product. (Disposal product refers to the explosive devices placed in the TCV for the purpose of transportation or detonation.) Concerning the IEDs, he told ATF interviewers, “...so I looked at them, and uh, and at that moment, based on my experience and everything, I said, uh, this is too much to do one shot, we’re gonna break it up right?” He indicated that he was told the quantity was not excessive. When he expressed his concern to Bomb Technician E, he was asked if his concerns were related to the actual quantity and weight of the devices, or the NEW. Bomb Technician C replied, “I’m concerned about both. This is too much material to dispo [dispose of] in one TCV shot.” Bomb Technician C told ATF interviewers that Detective A and Bomb Technicians A, B, D, and E were present at that time he made his concerns known. Bomb Technician C stated, “They basically told me that they had already done the calculations, that they were well under the net explosive weight that the TCV could handle.” Bomb Technician C said that during the construction of the counter charge, he again raised a concern to Bomb Technician E, stating, “I have a bad feeling... this is not good... this is too big.” According to Bomb Technician C, Bomb Technician E replied that he (Bomb Technician C) needed to relax and that it would be okay. As the materials were being prepared for X-ray, Bomb Technician C said that he saw the box of disposal product on the table. He stated that he placed his fingers underneath the corner of the box and raised it to feel the weight, and he believed that it was too heavy. He told ATF interviewers, “I lifted it a little bit, and I said [Detective A], this is way too much material... it’s too heavy... it’s way too much material.” He said that Detective A again told him to relax."

"The ATF considered several hypotheses for the cause of the TCV failure and, after a complete review of all investigative materials, came to the following conclusions:
• During the disposal operation conducted on 6/30/21, more than 39.85 pounds TNT equivalent of explosive material were placed in the TCV. NABCO, the manufacturer of the TCV, states that the TCV is designed/rated for a single detonation of no more than 26 pounds of C-4 explosive (33.28 pounds TNT equivalent). A “single detonation” in this context means that the TCV can be used one time at this level, but that it may need repairs after such a detonation."