Here is the problem, a driver is responsible for what their vehicle does 100% of the time. Example: if you rear-end a car because they stopped abruptly, you can feel like oh well they jammed on their breaks for no reason it is their fault. The legal reality is you are at fault because you were following too closely to have the reaction time and breaking distance to not hit them. That's why people who TRY to get you to rear end them for the insurance money get away with it most of the time. It is an exploitation of the law but reality.

So let's apply that theory to your situation. Let's assume you are correct that the bus stops abruptly. The van somehow manages to stop but you go past causing you to violate the law saying drivers must stop for school busses. Legally you are presumed to be at fault for not stopping. So unless there is some way for you to PROVE that it would have been impossible for you to see then you are responsible for passing the bus with its lights on regardless of what the bus did. As it doesn't appear you have that evidence then you have no defense.

Side note from the video, the van doesn't appear to be stopping violently. The bus is definitely fully stopped. As an uninterested observer you appear to drive past the van and the bus without an attempt to stop. That video is very strong evidence against your claim. Even if you are telling the truth that you honestly didn't notice till it was too late, the magistrate will uphold the citation because a reasonable preponderance of the avaliable evidence suggests that you were either willfully passed or due to inattention passed the bus.