In a Past Tense question, we use the auxiliary 'did' & the base form of the verb, e.g. "What did he practise yesterday?" We don't say, "What practised he yesterday?"
So why isn't 'did' used in Past Tense sentences such as: "What happened yesterday?"
The only explanation I could think of to give was that perhaps eons ago, we might have said, "What did happen yesterday?" but we don't use this form now.
Any explanation, theories, conjectures?