I'm thinking SD's theory is the major cause: In a hurry, not thinking ahead. I work to keep the time I plug a ramp slot to a minimum, especially on a busy day, with off ramp, pre-launch prep, and the same when landing.
Best is with two drivers, one to clear the boat off the trailer on kick off, and to drive on and hold on the trailer on retrieval. Wife's OK with the kick off, but won't back the trailer or drive on. So when I do the retrieval solo, wife holds the boat on the dock, I put the trailer under the boat and vehicle in FWD, park brake in fwd drive and winch on the boat. Mental check list is critical: park brake, trans in drive engine running, outdrive up in trailer. Then clear into the parking lot for final wrap up.