Why did insulation and vapor barrier cause mold in my storage shed?
Vapour barrier goes on the cold side of the insulation.

The problem here is that for an unheated, Un-airconditioned shed, the outside will always be the cool side.

Insulation won't stop heat transfer, it only slows it down.  A closed shed, no matter how well insulated, will always turn into a sauna.

Now you have a hot, steamy inside, and a (relatively) cool outside (especially at night).  If you put the vapour barrier on the inside, sandwiching the insulation in, the condensation will occur within your insulation. Hence, the mold issue.

You're better off without a vapour barrier in this situation.  This way the batts can dry out.   Even better would be some rigid foam insulation, but I really think that insulation in this situation is a waste of money.