Happened again today, but this time I noticed that the APU main swith was in the ON position when landing. I must have forgotten to turn it off after starting the engines. (You are supposed to let the APU wind completely down before turning it off, so I tend to finish the before taxi flow first, making it easy to forget).
I got the APU to start while taxiing in by turning the APU master off and waiting a bit, then trying again.