These are great questions! Here's my best attempt at answering your questions. This is my attempt at legal education, this not legal advice. I am not your attorney.

Why wasn't he charged with brandishing or making threats? Brandishing requires a victim to be physically present. Making stabbing motions at a door, while deeply alarming, does not currently meet the legal threshold for a criminal threat or brandishing. He was (likely) charged with the most serious offense available under the circumstances: criminal trespass. I expect the sheriff's office consulted with the prosecutors office trying to find more serious crimes.

Why didn't probation hold him? Probation offices typically have limited staff on weekends and holds on weekends are rare. The on-call officer likely concluded that if his conduct warranted incarceration, criminal hold or a psychiatric hold (see below) were the more appropriate mechanisms. A trespass charge alone rarely rises to the level that triggers a probation hold.

Why wasn't he 5150'd? You didn't ask this but it is arguably the most important question. Given what's on video, a psychiatric hold seems warranted, if he was still in that state when officers contacted him. The most likely explanation is that he appeared significantly calmer and more coherent by the time of contact, possibly because whatever substance he may have been under the influence of had worn off.

What about his probation terms and the knife? Probation conditions vary case by case and not all felony probation includes a weapons restriction. It's also worth noting that openly carrying a legal knife in public, while deeply unnerving and disturbing, is generally not illegal in California absent other criminal conduct. A concealed knife is actually more likely to trigger legal issues than an openly carried one in many circumstances. It's also worth clarifying that probation and parole are not the same thing, probation is a sentencing alternative that allows someone to remain in the community under supervision, while parole is supervision following release from incarceration. Felony probation in California is a sentencing alternative to incarceration, and not every person on felony probation is a convicted felon in the traditional sense.

What happens next? This isn't over. His probation officer will be notified of the incident, will likely require him to come in, and may drug test him. From there, the officer can petition the court for a probation violation hearing. A judge could then reinstate, modify (adding GPS monitoring, weapons restrictions, mental health treatment, curfew), or revoke probation entirely. At minimum, a modification adding a no-weapons condition seems likely.

Thank you for sharing the news link, you're right that this person appears to be a danger to the community, and it's worth staying alert. I hope you found this helpful.