Likely only a moderator would be able to offer an exact answer to this, but given that the relevant information would be covered by the confidentiality clauses in the moderator agreement a moderator would be prohibited from actually giving out any details. The descriptions available for the functioning of the software that detects this type of behavior are deliberately vague, to avoid people gaming the system, and additionally the software evolves over time.
That said, it is possible to answer this in the general.
Stack Exchange has numerous systems in place to detect unusual voting patterns, and some of these feed into the moderator reports. The most likely case here would seem to be that the user account has been giving out a large number of similar votes to a small number of users.
Such a voting pattern is normally a sign that someone has created a "sock puppet" account to boost their own reputation, or sometimes the reputation of someone they know. It can also be a sign of someone revenge downvoting or target downvoting someone in particular.
Because voting is supposed to be about the content and not the person, when the system detects this, it raises a flag for the moderators to look at. I don't know if it ever suspends user accounts based purely on software heuristics, but if the moderators (after investigating) agree that something fishy is going on, they can basically choose between:
- Doing nothing. Not really an option at that point.
- Sending a moderator private message. Usually a first step, to try to sort out the issue without resorting to suspensions. Moderator private messages are visible only to the users directly involved and to other moderators and Stack Exchange employees, and show up in the user's profile to those who have access to them.
- Suspending the user. This mainly becomes an option if private messages do not cause cessation of the inappropriate behavior, or if the behavior needs to be stopped immediately (the latter not really being the case in voting irregularities). Suspensions are time limited and the duration and cause must be specified at the time of the suspension.
- Deleting or destroying the user account. This is almost never an actual option in case of simple voting irregularities, but can be an option for example in case a user account was created to post spam or nonsense. Included here mainly for completeness' sake.
There is also the possibility of leaving a publicly visible comment on a post belonging to the user, but given how sensitive issues of voting irregularities can be, that should not ever be an option in such a case. It may however be an option in other situations, as a step before sending a moderator private message.