It was working fine last night, when all of a sudden all buttons became unusable.
I removed the battery, put it back, and now it works.

Any explanation?

No explanation, the information provided is too vague. Have you been able to consistently repeat the problem? If it keeps freezing, call Harmony tech support.

It just happened last night so no repeat so far. I was just curious if this is something that other users experienced.

I'm sure multitudes of people have experienced it, I know I have though rarely. It's just like a computer with Windows OS, how many of those have blue screened?

If the problem persists, see if you can repeat the problem by pressing a combination of buttons, starting a certain activity, or whatnot. If it's repeatable then at least you have a starting point to troubleshoot. If not, then it was probably just bad weather.