Although I'm not a bus driver (or indeed anything else to do with buses), I am employed as a van driver and sometimes pick up overtime away from my home depot.
The reason I do this is simple: petrol expenses. If I managed to get loaned out for a few days to, say, another depot in Teesside - I can almost double my daily wage through the petrol allowance I'm given which is paid in cash at the end of the shift. In addition to expenses, I also get paid for the time travelled to/from that location which, in the case of travelling to Teesside and back, is about an extra two hours on top of my normal shift. I also like exploring new places which is another reason why I jump at the chance to do overtime at another depot.
Perhaps bus drivers do overtime at other depots for similar reasons.
RE: GNE Overtime