MacBook Pro 13" (late 2011) gets extremely hot during normal usage
There's nothing significant that could cause your Late 2011 MBP to swell in that area. In fact, that area has a fan mounted there, which makes it quite unlikely that something could get hot enough to cause frame damage. 

My speculation:

If you've had swelling in the past then your frame is probably still bent from the swelling of the first battery. The initial heat and pressure could have caused the logic board to press against an opposite end. Furthermore, the heat damage could have created soft areas of aluminum that are slowly being modified by the heating and cooling caused by charging.

Here is the parts breakdown for curiosity sake.