What causes a wheel to seize?
It is rare for a bike bearing to suddenly seize.  Even a bearing "run dry" or exposed to saltwater or whatever will generally just get rough and squeaky.

In order to seize you generally have one of two things:


(Rare) A fragment of something -- sand, a sliver of metal, etc -- gets in between the balls in the bearing, causing them to lock.
(Common) The failure to properly fasten a lock nut or other fastener results in one of the bearing cones being "screwed" more and more tightly into the bearing, eventually making the bearing too tight to turn.


The good news is that the above second case rarely results in "fatal" damage (though it may have the effect of adding a few thousand miles to bearing wear).  So long as the situation is corrected and the bearing not "forced" to ride a considerable distance the damage is usually minor.