Here is the passage again:
Maria wasn't watching her plate of barbecue very carefully. So Santana,
the family beagle, snatched a chicken leg hanging over the edge.
As baked beans and potato salad slid onto Maria's
new sandals.
The third part is a subordinate clause fragment. This part has a subject, beans and salad, and a verb, slid, but the subordinate conjunction as keeps it from making a complete thought.