Whilst it hasn't been answered, it seems likely that he exploded due to his Hollow consuming his body. Aizen didn't have anything to do with the actual explosion as he was facing Shinji and duelling at the time. However, if Tousen's Hollow ate him because it was untrained and Hogyoku born, Aizen smirking at his death means he perhaps knew this would happen and therefore he had an indirect effect on Tousen's death. Of course this is mainly speculation.