Yes, it's redundant. "One reason may be" or "One possible reason is" are logical; they each have only one layer of uncertainty. "One possible reason may be" superimposes one layer of uncertainty over another and the result is too mushy and squishy to be logical. But all these expressions are grammatical, and they're all used commonly. "May" is very often injected into a sentence redundantly. You hear it all the time on US news broadcasts, for example. It's possibly because some people equate certainty with arrogance or impoliteness, and so they over-compensate. A single layer of uncertainty is appropriate; no more than that. :)

Can you give a bit more information and try it with a full sentence so we can fully understand the context?

Well in one sense yes because you already said "possible" which means it is not certain but you cannot simply remove the word "may" - so you can NOT say "One possible reason why people with higher income are not necessarily happier be because they work too much....". But you could replace "may be" with "is" .... One possible reason why people with higher income are not necessarily happier is because they work too much .....". But using "may be" sounds better imho. There is nothing wrong with some redundancy to emphasise a point.