I'm not sure it's about overworking as much as stress - stress is a known potential trigger for manic episodes. Typically in people who already have some biological potential for experiencing it. And the stress of having your partner come down with dementia and suddenly becoming their full-time caregiver must be absolutely massive.

But re the underlying physical thing: You might look back and be able to spot some episodes of hypomania in your uncle's life. Perhaps just reduced sleep, or twinges of paranoia, or periods of on-again/off-again hyperproductivity, etc.

Hope that if this story is recent he's doing well. The recent generations of medications to prevent and treat mania are a huge improvement on the ones available in the past, so fingers crossed.