The motor is suspended if I remember right. If one of the torque springs is worn or broken, it will give the issues your talking about.
Yep this is what they do before the spring breaks. Because the spring is letting the door essentially close past the limit on the opener, and then it retracts when it feels the resistance. It's probably only a matter of time before the spring pops.
I dealt with this working with them years ago, and I dealt with it in my garage about 2 months ago.