My first guess is definitely wet wood.
Lots of people, maybe even MOST people, don't have a good grasp on wet wood and dry wood.
They think if the tree's been dead for a couple of years, then the wood must be dry. Makes sense, right? (It's false.)
Or if they cut it into logs a couple years ago, then the wood must be dry. (Also false.)
Or if they cut the wood, and split it, and stacked it into a pile, it must be dry! (Could be, but if it's been out in the rain, probably not.)
So right now, you can look at the ends of the logs. If they're checked, that's a
good sign, but not a
guarantee. "Checking" is the word for the way wood develops these little cracks as it dries and shrinks:
If you want to know for sure, you need to measure. A moisture meter doesn't cost much, and it will make a huge difference in your woodburning life. Instead of
guessing and
supposing whether your plans are working (your
firewood procurement plans, your firewood stacking plans, your firewood burning plans), you can
find out and
adjust accordingly.
Here's a meter at Amazon for $11, with a five-star review.
Good luck! When you work the kinks out,
wood heat is glorious!