CiviEvent Overwriting Contact Info
Is it possible that you upgraded at the same time as you switched to WordPress?  Specifically, in 4.4 and earlier, events would match participants on email address only unless you checked the box to allow multiple registrations with the same email.  This was regardless of any dedupe rules you might have set up.

In 4.5 and later, an event will default to matching on the unsupervised dedupe rule, and you can choose an alternative rule on a per-event basis.  My suspicion is that your rule allows for false positive matches, and you haven't noticed the issue until the events started actually using the rule.

You might create a brand new individual unsupervised rule with the email address field, leave length blank, 10 as the weight, and a threshold of 10.  This will replicate the original situation of matching on email only.