Today is was driving an a mojor highway in Michigan (I94). The speed limit was 70 and I had autopilot on cruising at 75. Three lane highway and I was in the center lane. No one in front of me in any lane and suddenly the car slammed on the breaks for no reason. The guy behind me almost rear ended me because of my sudden slowdown for no reason. I immediately took it off cruise control (autopilot) and went on my way. The car had slowed from 75 to 55 before I could disengage autopilot. I have never had this happen before. The only thing I can imagine is that there was a large overpass coming up and maybe the shadow on the road ahead of me fooled autopilot? Any feedback or insights would be appreciated. PS. I did continue to use autopilot the rest of the trip without incident.