Why is my Early 2014 MacBook Air's Trackpad Uneven?
The most likely culprit is the battery being swollen.

When you get a physical "deformity" like this, it's due to one of two things:


accidental (or intentional) damage where the case and components don't line up anymore
something putting physical strain on the components themselves.


I'm assuming you didn't drop the Mac or have something heavy impact it.  

The battery swells because of off gasses being trapped and it's something you need to fix right away as the trapped gasses at a minimum can damage your Mac (what you're experiencing) to becoming a fire hazard due to thermal runaway.  

If you have the technical skills, you can remove the battery and continue to operate your Mac without it as you wait for a replacement battery to arrive. 

The battery is located just below the logic board - the four black "squares".  It's an entire module that can be replaced by removing the five screws (red/orange circles denoting different sizes.)  For full removal instructions, see iFixit.com 


Image Source:  Ifixit.com