DFS suddenly screwed up?

We had a problem this morning where suddenly, inexplicably after months of running with no issue, some of users were routed to the wrong file share via a DFS path.  Here's the setup:

We have two sites: Corporate (actually called Default-First-Site-Name) and DR.  In AD Sites/Services, we've defined these Sites by IP ranges - our 10.12.XXX.XXX ranges are Corporate and our 10.13.XXX.XXX ranges are DR.  In DFS, we a folder target (\\blah\public\profiles) with two paths: one for Corporate and one for DR.  The DR file share (obviously) is located on a file server in our DR site, and is a mirrored copy of the file share located in our Corporate site.

This has never been a problem, until today, when suddenly all of users reported really slow sessions.  Some quick research revealed that users were drawing their profiles from our DR site thru the DFS path for some unknown reason.  Nothing had changed in the configuration in months.  We disabled the DR path in DFS, had everyone reboot, and all was back to normal.

So, how can this happen?  My understanding is that when you logon to the domain, you'll be assigned to the appropriate Site based on your IP, and then you'll resolve your DFS paths based on your Site, full stop.  Is there some sort of load balancing going on underneath the hood that I don't know about that might elect to pop someone over to a different path that isn't supposed to be available to that Site?

Thanks!
Matt