Depends how the BB has played previous hands and how high he has valued top pair.

He may have felt he had odds preflop to call with a marginal hand with 3 others already in for another 80 chips, so it's hard to put him on something specific.

He could have anything from TPTK (A9), 2 pair, a set or a flush draw.

I'd need to know your stack sizes to comment further.

Tough decision.