How do scopes get off over time??

So, I recently (last year) put a decent(?) Nikon Pro-staff scope on my Savage Axis rifle in .223 and had it sighted in pretty well. Fast forwords to this last weekend. I checked the zero on this scope/rifle and it was 3 - 4 inches off!

How does this happen on a good brand name scope?

how often do these things go out?

I have a cheap BSA varmint scope on my .308 and its never gotten out of zero....

Thanks!

How do scopes get off over time??