One contact clicked a link thousands of times

I am wondering if anyone has seen this behavior: we saw a spike in traffic to our website, but had not sent out a marketing email on that particular day. After some investigating, I found a handful of contacts who's activity showed thousands of clicks to links in an email. The email in common happened to be a nurture email that has already been sending for a few months, and the links being clicked within that email varies. None of these contacts are "new," in fact, many are established, semi-engaged contacts. I will also note that out of around 20 of these offenders, there were 2 groups of 6 that shared the same email domain.