It probably was a third-rail cover failure. The covers are lengths of fiberglass which are joined together end-to-end to cover the rail.

The joints are made by over-lapping the ends of the pieces, and locking them together with plastic push-through connectors called "darts".

After years of exposure to sun and weather the darts can fail and a piece of cover can drop down.
If the end is facing the direction of travel, a paddle can hit it and break off. (Actually, a BART engineer told me that it can break off all the paddles on that side of the train...).

The covers should be installed like shingles with the joints running one way so the paddles would lift up the fallen cover rather than hitting the end. In some cases this wasn't (or maybe couldn't?) be done.