I did a pay for delete and when it came off my score dropped. Also, I've had my score drop when other derogatory items have fallen off.
A collection is something that will banish one to the derogatory scorecards.
It's possible if there isn't much positive history, and say a 30D or 60D late or whatever on file (two different resulting scorecards), that because of the way the scorecards overlap it's possible to move down in points even if you're moving to a "cleaner" scorecard.
Will state though that if you have multiple collections, and one of them comes off, that will not result in a score drop.
The majority of times it's not rebucketing which causes a drop, it's something else that changes in the file but most people's files just aren't constant enough to really tease out every single change.
This happened to me today. I'm not new at this. I looked at every line item and the only change was a medical collection was deleted. My score dropped 28 points in the past 7 days. WTH?