Road vehicles stop all the time for all sorts of reasons.

Safely coming to a stop because the conditions are unexpected is the right thing to do for an autonomous car.

Slow response by exception handlers is more of a problem.

Freezing and not moving is also a very human panic response.

As opposed to what?... Floor it? Just stopping is the safe default* that should be practiced by more human drivers, too.

*) In the general case especially within cities. There are exceptions - for example on roads with high speed limit like euro highways, where stopping for non-emergency reasons is banned anyway.