sounds like just checking in the wood
Checking is a separation of the wood fibers along the grain. This occurs when wood is pressure treated or seasoned, as the fibers expand and contract and internal pressures build up. You can think of checks as small earthquake faults that have shifted in order to equalize internal pressures.
The larger (dimensionally, not in length) a piece of lumber is, the more likely it is that this will occur. I am unaware of any method to prevent this. There is a picture and description of this and other “normal” defects here:
