I am curious if it was a UA or CO operated flight. I have seen a few posts that seem to indicate it's more prevalent on CO operated flights. I've had it happen on a quite a few flights from EWR-south Florida-EWR but I fly that route 30-40x a year so it's probably not a fair indication. I understand when there is turbulence but I've experienced it on perfectly calm flights. Never more than a few hours though, I would get up if it was more than an hour or two.