Can it be he just doesn't care about the work assigned to him so he just half-asses it? Not saying it is a good excuse, but I have seen good developers that are

Can it be he just doesn't care about the work assigned to him so he just half-asses it? Not saying it is a good excuse, but I have seen good developers that are indeed good, when moved to projects they have no interest at all, become like your co-worker.