I noted that some reputations of mine was removed due to cancellation of some user account.
- What can cause a user cancellation?
- If a user is removed, is all their traces removed too?
- How usual is this?
The cancellation was caused by repeated infringing of rules.
Cancellation of real users is a very unusual occurrence.
Edit for clarity:
Except in the case of SPAM accounts and sometimes sockpuppet accounts (used for artificially inflating reputation), moderators don't (at least as far as I am aware on Math.SE) delete user accounts unilaterally. So to clarify Mariano's answer in the particular case that prompted this question:
Hence (indirectly) the account deletion was "caused" by repeated infringing of rules. But I would be more callous and say that the real reason is the user disagreeing with the notion that there are rules and there are consequences when rules are not followed.
The most common cause for user deletion is probably spamming, new users that only posted spam on the site and nothing of value are often just outright deleted. The other possibility is that a user requested self-deletion of her/his account.
If a user is deleted, their posts with a negative score are removed, all their other posts remain. Their usernames are anonymized and their profile is completely deleted (not a soft-delete like it happens for questions and answers here). Their votes are also removed.