There are many possibilities. I'd, personally, refrain from using the words "tampered" and "attacks" since they carry a moral implication.
It also seems it could be that a name based virtual host configuration changed mid session. I suppose this could be considered routing in a sense as well.
Again, the fact that it hasn't happened often and that your memory/knowledge of the exact transaction of packets is likely incomplete, makes all this just speculation.
My guess is it's probably just a glitch and nothing nefarious is going on. I wouldn't be too worried.
It's good you're keeping an eye on things though!