Long downhills at 7-10% grade and hanging on rim brakes on the brifter tops will do that to me. You squeeze them between your thumb base and the remaining four fingers. Switching to disc brakes and the much lighter pulls needed has made it a problem of the past for me. Also, on downhills get into the habit of leveling your cranks with the outside (depending on the next upcoming turn) crank forward - then try to hold your weight back with that forward foot to offload it from your hands and bars. It will likely require scooting back slightly on the saddle, and on the steepest sections (15-20%) getting behind it altogether. (This isn't as awkward as it sounds since on dowhills the bike rotates forward around the cranks, but it's also why you easily get a ton of weight on your hands.)