Constancy of Temperature

Significant variations in permissible temperatures, if allowed to occur regularly, can be as damaging to wine as impermissibly high or low  temperatures.
An increase in temperature increases relative pressure within the bottle, which may force air (or in extreme circumstances wine) past the cork. As a bottle cools, air with fresh oxygen may be drawn back into the vessel.

Oxygen, another enemy of wine, is a well known chemical catalyst. Catalysts speed up chemical reactions, and in the case of an aging bottle of wine, that is not good. 

Infrequent slow oscillations in permissible temperatures, such as slow seasonal variations in a passive cellar, can be harmless, though repetitive temperature variation occurring in all but the most occasional of circumstances, is for these reasons to be strictly avoided.

Maintain temperature constancy. Regular variation in temperature of no more than a couple of degrees is permissible, though not ideal.