Honestly, I don't think this rifle has enough recoil for bump fire. Combine that with the heavy trigger pull, and boatload of creep, it's very unlikely that it bumpfired. Not to mention the reset is too long. If it did in fact fire more than one round on a single trigger pull, there is only three basic possibilities.
1. you experienced a slam fire. While this should not happen on this rifle since the firing pin is pretty heavily sprung (not free floating like a real AR). I suppose that a piece of debris on the bolt face, or around the mouth of the chamber could set off a round if it hit just right. But that's not likely either.
2. Hammer Follow/Trigger disconnector failure. If the disconnector fails to engage the hammer when the bolt goes back, or the bolt does not travel rearward enough to make the hammer engage the disconnector (but I don't think this is possible on the 15-22) the hammer will follow the bolt forward and could still hit the firing pin hard enough to ignite the round. It's very possible with a .22 LR that the disconnector is fouled enough to bind up and not engage the hammer when it comes back.
3. I just had to go after my 1 year old who was trying to feed her dinner to the dogs, so I lost my train of thought and can't remember the third possibility.
Regardless of the reason, it is a condition that you really don't want to mess around with. If your rifle doubles, you need to stop and find out why. There are very serious consequences if you knowingly allow this condition to exist on any firearm.
