Well I must say you are lucky to be getting a replacement considering it was your fault. You know the refresh rate and you went beyond that.
First you say this...
I am running mine at 70 right now been that way for a week now. Gonna change it back to 60 since there is no difference. I was just messing around with an app for my Mini.
Than this..., pot meet kettle?
Nothing is going to force a different refresh rate every now and again. The chances of that happening are slim to none. If I am wrong please provide me with data that says otherwise.
Slim, but not none. It happens, it's rare but there is always a chance, be it the scenario I explained, or a game unknowingly forcing a resolution/refresh rate, or as the example you stated, in normal day to day windows operations it's unlikely, but not impossible.
Quite frankly I was never convinced the monitor broke because of the refresh rate, it worked after it happened, however that was the only reason that came to mind, considering you say you've been running yours @ 70hz for a week, I'm even less convinced mine broke as a result of the forced refresh. I didn't tell them them it was hooked up to a computer because they would immediately assume the worst, if they examine the monitor and find it was my fault than so be it, but I didn't knowingly change the refresh rate like you did.
I won't go into details of how it's broken, but to me it seems more like a software glitch.