Smooth Sumac leaf-shed and replace?

Why did a couple of Smooth Sumac trees (that are planted close to the south side of the house as summer shade) drop all their leaves and promptly grow many tiny ones from all over their trunks?
The new ones are regular leaves but remain sort of miniature versions of the full things.
I could see dropping all the leaves and dying, but not dropping them all and then replacing them.
(The trees are very lightweight and so make nice close-to-the-house porch-like shade trees ; also they grow fast. They do have a tendency to drop things though, like half a trunk at a fork, or leaves.)
Ron Hardin
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