I was traveling on the DC beltway (interstate), it was raining and the temp was in the low 50s. The vehicle in front of me hit their brakes pretty hard. I started braking firmly myself. I heard an alarm sound and I believe the anti-lock braking activated, and the van began automatically braking itself. I was brought to a complete stop on the interstate by the van. The vehicle ahead of me accelerated again and I stepped on the gas to get going again and it wouldn't move. At all. I kept pressing on the gas to get going and the van wouldn't move. Meanwhile traffic behind me is gaining on me and I was rear-ended. After being hit I tried to move it out of the travel lane and it still wouldn't move. After all that I looked at the dash and it said to move forward, release the electronic parking brake (or something to that effect). I pushed the button to turn it off, and was able to get onto the shoulder. I believe the root cause of the crash was that the van automatically turned on the parking brake on it's own, and I did not want it on. If it hadn't, I could've accelerated away and not gotten rear-ended.