It's just business sense: if the previous guy drops the client like a hot potato, it's a pretty strong hint that they're trouble. Although in SBF's case you don't really need to rely on subtle hints to find that out.

lawyers in California are in a register, available to others in the profession. Over time, law practice diverges significantly, and the norms of those specialties are partly cultural and partly for saving-face or other sorts of soft-values. The truth is that half of what lawyers do involves serious-life-changing winners or losers, so there is a thick skin of indifference among many. With all that said - certain specialties of generally-rivalrous groups do keep careful track of the opposition names and faces, probably less careful track of ally names and faces. Some of those lawyers have a very, very good memory for these things. $0.02