Odd things found with chainsaw

[underdog]

So I was watching a tree removal video and they went thru a bunch of rocks, dirt, nails etc. 
By the end of the tree, they claimed one saw of many still sharp.

Anyway. 
It reminded me of something that happened here.
We were clearing some small trees. Backhoe and chainsaw. Had good help, so I was on backhoe and the help was cutting.
This one tree went off like a blown truck tire when he sawed into it. I heard it over the noise of backhoe.
Helper was slightly shaken, but ok and finished the cut. The small tree, about 16" to 24" across had a hollow at base. 
It must of had some pressurized air in it as we found no foreign objects, just the hollow.

My mind puzzles this occasionally.
Ever seen anything like that?
How could that be possible?

[Magicman]

It may have been filled with methane, etc. and the chainsaw bar/chain produced a spark as we have all seen them do.