Here is the passage again:
As quietly as possible, Sherri tried to open the one-pound bag of candy
on her lap. Finally, the tough plastic split open.
Causing an explosion of chocolate balls that bounced
and rolled down the aisles of Dr. Wilson's chemistry class.
The third part is a participle phrase fragment. Every sentence must have a main clause, which will follow this pattern:
Subject + verb = complete thought.
Sadly, this part does not.