Or........perhaps they auto-retracted because of a truck tilt disagree, or wheel-spin up signal loss/out of limits to the spoiler arming and control system. The aircraft came up on the struts a bit after touchdown, and may have gone far enough to "un-tilt" the trucks creating an air-ground dis-agree, perhaps indicating to the aircraft systems a potential go-around. They retracted when the nose wheels went on, which for the pilot flying would have been a bad time to be working the speed brake handle, and distracting having the pilot not flying reaching around the reverser handles. Maybe................?