Why only one upgradable lock can be held with shared locks

The boost documentation for upgradable and shared locks says that when shared locks are held only one other thread may acquire an upgradable lock. So that if other threads try to acquire an upgradable lock when shared lock(s) are held along with the upgradable lock they will block.

Is there some deadlock possibility that I am missing when more than one thread acquire upgradable locks along with one (or more than one shared lock)? Or is this just a logical requirement (so a "should not do this" sort of thing)?