Endermen aren't careful enough around water (or lava) and sometimes run straight into it. I believe this is new behavior. 

It might also just be because of it's speed, and any other mob would fall into lava if they were as fast, but it's still probably not intentional as they are supposed to avoid water and lava.

How to reproduce:

Aggro an Enderman and go in a pool of water, where the edges are like in the attached examples. The Enderman needs to run off a higher block to drop into the water.

Punch the Enderman to make it teleport, and move around, to get it run into the water.