Sounds like you are just releasing the built up tensions in the log, these can be in the tens or even hundreds of lbs per sq. in. so causing it to pull itself apart when you upset the balance.
This is always a dilemma with a log, the temptation is to try and use it whole so that you can get the biggest item out of it, but this invariably fails for one or more of several reasons.
Next piece, try splitting it in half through the core and turn something smaller out of each half, even then you may have to loose a portion of the wood near the core due to its tendency to split from the core point at the bowl rim.
If you can't turn it very thin whilst green, and suffer the distortion that will occur, then you will need to slow the drying down of the rough turned piece as much as possible.
