Lens guard question

Today I was using the premium lens guard for the first time.

I made sure that the lens was clean, and the camera + lens was completely dry.

After a few minutes filming, the lensguard was condensating, fogging on the inside. I removed the lensguard waited all to dry up, and placed the lens guard again. It started to fog again very fast.

I never use my camera into water or rain. I am sure it was as dry as it could be.

The weather was very warm, dry and sunny today. Camera became quite hot during filming.

After taking off the lensguards, I could use my camera as normal.

What went wrong? What can I do to prevent fogging inside the premium lens guard? My camera was dry already.