Here's an even simpler hypothesis: Some sort of memory effect
is involved.
Think about how the thing was made: It started out as a ball,
then a hole was poked in the middle. Poking adds material to
the inside of the torus, and puts the outside under tension.
The inside is either under compression or under much less
tension than the outside.
To test this hypothesis, construct the bagel using some other
technique, perhaps by assembling small bits, or perhaps by
cutting with a donut cutter.