On my flight from SAN to IAH last night, I was upgraded at the gate after being #2 on the UG list.

Several minutes after boarding, I was told by the FA that I needed to move back to my original seat with no explanation.

I believe I've seen this story before on FT, but wanted to ask how often this occurs? If so, what are the reasons, and is this legal based on their TOC?

I did only pay for a coach ticket, so in no way am I complaining. I'm just wondering why/how often it happens.