I started making mash ups a few years back while I was showing someone how to use Ableton Live - it seemed like a fun way to show it off. That's how the Ik vs Bad mash up came about.
Other mash ups just happen when I'm a bit bored and I fancy messing about with some tunes.
One interesting one happened when I was a student at Salford Uni and we'd moved into a house that had loads of old Indian records in one of the top bedrooms.