Water pressure increases by .5 psi for every foot of depth. More pressure = more force pushing it through the dam. If there was a little seepage along the length of the pipe going through the bottom of the dam, more psi = faster flow, which will cause a washout quicker.
Here's something that NRCS published in 2011, that still has anti-seep collars although they say what you are describing is a viable alternative. I'd really like to hear Mike Otto's thoughts on this.
NRCS Code 378
