You probably see this happen most frequently among novice divers who are overweighted and/or are wearing a very thick wet suit.
The diver requires more weight to overcome the inherent buoyancy of the thick wet suit. As the diver starts descending the neoprene compresses. The neoprene compression undergoes its most severe buoyancy loss at about 15 feet (5 m) or so, depending upon the suit (this is one reason many divers have difficulty maintaining depth at the safety stop).
When the wetsuit compression kicks in and the extra buoyancy is lost, the additional weight required to overcome the inherent buoyancy results in a very rapid descent. The diver is generally caught unawares of this buoyancy change and doesn't anticipate it with incremental additions of air into the B/C. The result is a very quick drop until the diver can get control of the descent with his/her B/C.
That said, what generally happens next is the onset of an unexpected/unwanted rapid ascent due to the fact that, in order to stop the descent, the diver has put too much air in the B/C too quickly and has become positively buoyant.