What would cause this?

Termite by product form the great meal on you!!

Hi Cinda, it should have been treated since it is out in the elements and should be resistant to wood eating pests so I do not know what would have caused this.

According to the internet, pressure treated wood is insect resistant not insect proof. If the wood stays wet long enough to start rotting, termites, carpenter ants and the like will start to chew. It has been a soggy year. . .

Look and see if you see a perfectly round hole (about the size of a dime) in the wood above where the saw dust is at. If so, most likely it is wood bees. They look a lot like bumble bees but are not aggressive to humans.

Termite make tubes from the ground up and dig into the wood. Eat wood from inside out. No wood dust like you have. Libby may be correct. Wood bees or carpenter ants.

Pressure treating is a preservative process that helps wood withstand the elements and wards off termites, microorganisms and fungal decay.

I think there is also a type of wasp which does this too. unsure about their aggressiveness? my neighbor just had landscapers attacked by swarms of bees or wasps? (long island, ny) simply raking her backyard....lots of dead wood lying around --but no one stayed around long enough to see where they came from. one worker ended up in the ER. :( she has someone coming in to try to remove them.

Yellow jackets are very aggressive - they live in the ground. Quite a few years ago we had to have an extermination spray the front yard, as they are dangerous and bites can, indeed, send someone into anaphalatic shock. After that one treatment, they never returned and it's been years now...Not something to fool with...