What's happening is that the ECU is introducing too much fuel to the engine in an effort to burn off soot in the diesel particulate filter. That excess fuel is dribbling down the bores into the sump, contaminating the sump oil so it no longer lubricates properly and raising the level to a point where the engine could stat running uncontrollably on its sump oil. I'd guess you're probably not driving far enough after each start-up and would be better off with a petrol engined Yaris or a Yaris hybrid.