It could be from the heat, but it could also be from residual
moisture in your deck boards. If you're applying stain over
PT wood, it's moisture in the wood. I had the same problem.
If it's composite decking, it's the heat. You could break
the bubbles and 'spot prime' with the stain. Let it dry, and
apply another coat. If you have excess moisture in your deck,
there's not a whole lot you can do. It needs to spend several
days in a kiln to dry out the moisture. This was told to me by
a guy who worked in a mill that utilized a kiln for drying
for drying lumber. Hope this helps.