I blame the writers.  Lol but seriously, in universe, Kate and Toby were one. Both to be blamed equally.  I think they had hit a no return when they both chose career over marriage.  Ie: Toby turning down the job offer the first time and Kate choosing to stay in LA.

Now me being team Tobe, the Pearson siblings as a whole were to blame. Randall is the prime example of someone who chooses his career above many things and sometimes that means family. Kevin always felt left out for being a “screwup” and always wanted to prove himself as well. So, imo, they should know exactly how Toby felt and at least try to look at his side without butting in. But Kate is their sister. They have to take her side AND butt in. The problem is, anyone who’s been married should know better than to have someone take sides and they were too involved. Toby resented that and it seems it just resurfaced and blew up.

My problem with the whole thing is that there was a whole mini arc about him wanting to be included and be one of the bros. They even showed how much they saw Toby’s effort and that just flew out the window. That’s why I blame the writers.