It just seemed strange. It didn't seem like the sort of stuff that would come out of a grease impregnated stock just because it got wet.
Oil floats on water, so it may have been the lighter oils separating from the grease or cosmoline. Also, it may not have been entirely due to the water. How warm did it get where the rifles were stored?
One of the popular ways to blot cosmoline out of old military stocks is to put them in a large black plastic garbage bag with plain clay kitty litter (not the clumping stuff). Tie the bag closed and sit it outside in the summer sun for a week or so. The heat will make the grease ooze out of the wood, and the kitty litter soaks it up. Works like a charm, especially if you remember to rotate the stock inside the bag to promote even heating. There's a stock refinishing forum on the "milsurpshooter" website which gives more details and suggestions.
Brownell's also sells a finer-grained powder called "whiting" which works the same way.
Beware of the oven cleaner method; it tends to leave light-colored wood with a slight greenish tinge.