Suspensions go to people who break the rules of the site, are warned about it, and don't stop. There is no requirement that the user does not contribute to the site in a valuable way - or in other words, even if someone is making valuable contributions in asking, answering, and editing, that doesn't make them immune from suspensions. No amount of rep puts you above the rules.
Specifically, "voting irregularities" typically means the person used multiple accounts ("sockpuppets" as we call them) to vote or participate in a way that they wouldn't be allowed to using one account. For example, voting multiple times on the same posts, voting on their own posts, using multiple usernames to give the illusion of broad support in a discussion (most relevant for meta posts), and so on. You can't really trust the reputation of these users because some of it may have been earned invalidly, by voting their own posts up, or by sockpuppet accounts awarding bounties to the main account. There is some more information on this on Meta Stack Exchange.
The normal progression of suspensions is that you first get a warning, then a 7-day suspension, then a 30-day suspension, then a 365-day suspension. So when you see someone who has been suspended for a year, that typically means they have been told to stop what they were doing at least 3 times in the past, but they didn't. Now, those are just guidelines, not strictly enforced limits on suspension length - sometimes we adjust the suspension length, or skip the warning in egregious cases (e.g. voting irregularities with new accounts, where the poster doesn't have an established record of good contributions) - but we rarely deviate from the default procedure, and I don't think there's ever been a case on this site of a user being suspended for 365 days without going through the full sequence of prior warnings.
Whenever a suspension is issued, it is accompanied by a message which usually looks like the following (taken from my answer on MSE):
Hello,
I'm writing in reference to your Secret Agent Stack Exchange account:
http://spies.stackexchange.com/users/007/bond-james-bond
Due to clear signs of falsified accounts being used to artificially
inflate your reputation, your account has been temporarily suspended
for 7 days.
It is not fair to other users of Secret Agent Stack Exchange to allow
users to gain reputation through sockpuppets.
The offending accounts have been removed and the votes invalidated.
Please refrain from this behavior in the future.
Regards, M
Secret Agent Stack Exchange moderator
Whenever relevant (e.g. in cases of rudeness), we usually quote 3 or 4 examples of the offending behavior in the message itself. Warning messages are usually similar, except for the part about actually suspending the user.
My question is: Do I have to worry that this could happen to myself?
Why, what have you been getting away with? :-P But seriously, I hope the above will convince you that you are not going to get suspended by surprise.
To clarify: I don't mean to interfere with moderator activity and the punishment is probably appropriate, but it would be helpful to understand what's going on.
Indeed, people are welcome to ask about the suspension process in general, as long as specific users and specific suspensions are kept out of it. The system is designed to avoid public shaming. For that reason, we (mods) do not discuss specific suspensions, and we prefer that other members don't either.
Finally, all suspensions are audited by the SE team. If you believe something inappropriate is going on with a specific user's suspension, you should contact them.
