Yeah, like rbow said, it's got to do with a fatigued bench.
Set your player hierarchy for DH with all of your fatigued players (and another fatigued player as your starting DH) and set their autorest to 99. Sparky will skip over them, and as long as you don't have any other hitters on your bench sparky will start one of your pitchers as a DH. It's easier to do if you draft your team with this in mind, as you can draft the minimum number of bench players and all with low PA so they fatigue quicker... but it is doable. You can do this same thing at other positions to get Sparky to start pitchers at other fielding positions as well. I haven't really paid attention to which pitchers Sparky chooses to start, so I don't know if it's based on PA or some other factor.... so don't think you can control which pitcher is (beyond the one) Sparky will use as his goto pitcher.