Well, it's certainly difficult to speculate as to the causes of these past events. I can think of a slew of reasons you may have had the experiences you mention; ssl stripping being just one of them.
So, without being able to reliably reproduce the problem, I'm afraid your only recourse will be to stay vigilant and... have investigation tools ready (e.g. wireshark) for the next time it happens.
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!
I understand it's "speculation" since I don't know the exact mechanism or have proof as to what happened, but it's very far from a glitch. Again, this happened a few times on multiple different physical computers and didn't set off any alerts in my security systems.