I do not remember having seen pop tarts anywhere, though no doubt I have passed them in shops from time to time. As nearly as I can make out from the Web, they seem to be biscuity things, and not necessarily symmetrical. Accordingly:

1: In general irregular or asymmetrical  expansion and contraction are what cause distortion.

2: Heating at toasting temperatures causes shrinkage from loss of water in some parts of the substance, and expansion in other parts when steam from the hotter areas moistens them.  Hence bending.

3: When sugary, doughy material material is heated till it crisps or browns, it distorts and sets permanently into a new shape.

4: A toaster cannot generally heat both sides exactly symmetrically, and even if it could, putting a slab into the toaster usually will bring it closer to one side or another.

5: Some toasters heat first one side then the other. This begins with a maximal bend one way, followed by a porly-matched bend the other way.  You sometimes see that sort of thing  with steaks at a barbecue.

I'm not familiar with Pop Tarts either (though, unlike Jon, I knew of their existence). Is it possible that the product is manufactured deliberately to exploit the effects Jon mentions so that the "bend" is characteristic? I can just imagine someone in Marketing basing an advertising campaign on something like this!