What causes one to be sealed though?Complaints can be a function of where you work. The local thuggery will complain you're "harrassing" them to attempt to get the brass to call you off, intimidate you over the chance of a lawsuit, etc. Or, they can be completely legit and the officer screwed up. If you work in certain areas (and I do mean work, as opposed to be just assigned to) you will get complaints.
I've certainly been complained on for things I did not do, including an allegation of a serious felony by a mentally ill woman who probably really believed what she was claiming. I've also been complained on for things I absolutely did do...but the person didn't like it despite it being within policy, within the law, ethical, etc. To date, I have no SUSTAINED complaints in my file, which is really what matters. Remember there is no burden of proof at all to file a complaint, and in most places there's no repercussions for filing a false one even if it's technically illegal.