IMHO when you 'drilled' with the nail all you did was drive a nail into the
wood twice, once with the DP and once with the hammer. Each nail spread the
grain in the timber until it reached the breaking (splitting) point. When a
drill bit drives into wood it removes some of the wood, so you should have a
hole in the timber with no pressure in any direction. Then when you drive
the nail down it will exert enough pressure sideways to retain the nail but
not enough to split the wood.

To avoid splitting you should pre drill, not pre nail.

Mekon