Why are there air(?) bubbles within my LCD?
There is a piece of glass on the front of the device (called a digitizer) and then the LCD is sandwiched up against that glass.

What your picture looks like is that something has gotten in between the glass and the LCD. You could probably fix it by opening the device and cleaning/re-seating the LCD against the glass.

As to how did it happen, difficult to say. Possibly what ever glue or mechanism that holds the bottom of the LCD to the glass has come loose. Maybe from impact/shocks or just age. If it's fairly new, then I would start thinking about manufacturing errors.