Some of you have touched upon this - I wasn't a party to what happened Saturday - I can however speak from experience:
Why would they do this on a crowded Saturday?
Because they wanted to make an example / make a statement / get Disney's attention.
Why would they need to make an example / make a statement / get their attention?
Most often, they do this because: a) the inspectors feel you are not respecting them, for example by blowing them off; and/or b) someone disrespectfully mouths off at them.
There are a thousands things any business could be shut down for on any given day, but if you are showing them respect, and that you are trying in good faith to address their concerns to help keep things safe, they will work with you.
If you have done something to lose their trust (like in my example: ignore them / mouth off to them), then sometimes they will "make a statement" to force you to respect them and remind you that behaving in a safe, professional, manner isn't optional.
Reminder: the reason they exist and are there in the first place: the public complained and asked their elected officials in the California State House to do something, because of deaths, in more than one park (arguably because profit was being put in front of safety). New laws were written as response, and new levels of supervision on the parks is the result. The California amusement industry has no one to blame but themselves.