Grooves worn away on seat post

Hubby got home today with a dollar between his saddle and seat post. Apparently, while he was riding, his seat got really loose. When he stopped to check on it, he saw that all of the grooves on the carbon seat post had been worn away. Any ideas as to why this would happen? We bought the post used off of ebay about a year ago, but is was in like new condition with no wear marks on the top groove. I've attached a picture. It is of the post. The metal clamp that mates with the top of the seat post is worn too. Thanks!

The screw holding it down was likely not quite tight enough so it rubbed away at the top. I had that happen on a post once. I switched to a post with a metal top to it.

Did you switch to a carbon post with a metal top? If so, what brand was it?

That can happen on aluminum posts also, if they are ridden loose.