It likely has to do with the magnets. The magnets are "magnetised" with the motor assembled, and pulling it apart (pulling the magnetic fields across one another) likely scrambles the individual poles on the particles enough that it weakens the magnets. That's my guess
I know the same thing happens with a magnetic dial indicator base. Disassemble the magnet and it becomes weak as hell.
In the case of the motor, you might be able to take the endbells off ( if it is built that way), leaving the rotor in place inside the stator, and you might live to tell us about it
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)