My guess is that a defective cell produced huge amount of gases when the starter was running. It was because hundreds of amp of DC current went thru the defective cell electrolyzing the water. If you jump start you car, that battery may blow up in front of you.

My guess may not be correct but it is good idea to keep a distant from the diabled car during the final step connecting a jumper cable to jump start the car. Take all the precaution as suggested by the manufacturer.

David