Some are explicitly called "intern" or "PT" (part time), some very much look like probable contractors, but I don't know how contractors end up with contributions to insurance/pension/etc, and others I see no obvious reason for such numbers at all.

Conceivably some of it could be attributable to oddities of recently-retired/fired/resigned employees, but I'm sure not all of it.

I think this dataset is impossible to make meaningful sense of without much more information on BART's employment practices, possibly wider laws/regulations on California public employees, and more specific information about individuals.