OP, one indicator that you changed buckets is to look at the 2nd and 3rd page which lists the things that help and hurt your score. If those are out of whack and out of order between pulls, then that can very likely be the culprit. For example, #3 is now #1, or vice versa, or there are new things added.
If not that then I'd look at these in the following order:
1) Look at util. Specifically, make sure individual CCs remained in check and that there's nothing with a higher balance, or even maxed out.
2) Look to see if there were any added TLs...or missing TLs. A new account can tank your scores. A dropped account can hurt if one of your oldest.
3) Compare each positive and negative factor in the before and after reports. Make sure there's nothing along the lines like a recent late mention, new accounts, added baddies, etc (via "time since late late" if showing).
4) Look at the total number of accounts on the "Credit At-A-Glance" page and make sure there were no added red flags (or missing as far as that goes). Make sure there were no added accounts in the past year (via comparing the numbers), no added inquiries, no added CAs, PRs, etc. Make sure that nothing is listed past due that wasn't there before. Make sure there aren't added accounts with derog references, etc.
5) Look at each individual TL. Look for the small differences, especially comments, "Worst Delinquency", balances, and especially CLs. For example, DSNB (Macys and Bloomies) removed their CLs and that impacted many of our scores. Look for other "upgraded" cards that maybe you really didn't want upgraded. Some CCs have been switching to signature cards and that can impact your score.
6) If you have any baddies, like a CO or CA, look to see if any updated between pulls. A change in date can drop your scores if old now new.
7) Finally, if all else fails, revert back to rebucketing.
