I think it happens most with G and B string because those are the strings most folks bend the most. I agree the machines probably aren't the problem, but don't discount the importance of proper securement to the peg. Everybody's probably going to tell me this is wrong, but a friend showed it to me many moons ago and I've been doing it ever since with no problems. When you put the string on, slide it through the hole, leave enough slack for a couple of wraps, and tthen immediately on the other side of the peg wrap the free end of the stringin the opposite direction of how the string is going to be wrapped around the peg. Go one half a turn so that the free end comes back around to where the string is going into the peg. Slip the tail under the part of the string going into the peg and then pull it straight up. Now as you tension the string, the first wrap will sit on the tail effectively tieing something of a knot so that the string cannot slip. Tune up as normal, stretch as normal, etc. Don't know if I explained that very well... Anyway, not a 'necessary' thing by any means, but there's a lot of tension on the strings and the unwound ones are really quite slippery. This method ensures no possibility of slippage on the peg.
J