Max operating temp of IC's
All semi-conductor characteristics are affected by Boltzman statistics relating charge carrier densities with respect to temperature.  The hotter it is the more intrinsic carriers are present, at some point the intrinsic carrier concentration gets so high that any doping (n-type vs. p-type) gets wiped out.  That is at high temperatures.

A conductor has the characteristic that as you heat it, the carriers are more mobile and collide more and resistance goes up.  A Semi-conductor has the characteristic that as you heat it up, more carriers are present and the resistance goes down.

So it's natural to see that there are limits.  Why particularly those temperatures, I don't know, I'm sure some one will come up with the historical answer.  However, it's very celar that some temperature must be selected, because if you design for a very broad temperature range then some other performance metric will be compromised, like speed or margins.

Designs are specified over what are called PVT corners, as in Process, Temperature and Voltage corner cases.