What is the purpose of irregularly shaped SMD pads?
I saw many datasheets and created many custom devices in EAGLE, this one is not much complex and does not involve many calculations although I tend to agree with commenters to your question that there might be better ways to describe the part's footprint.

Please notice the caption under the drawing: it says mounting rather than board footprint or whatever. This shows shape of soldering area, not shape of the pad itself. Big areas are intended for close contact with button's pins, while small perpendicular areas seem to be designed for extra solder "pour out" space.

You can make pad of any shape given the areas drawn on the mounting schematic is not covered by the solder mask.