Chain slipping of middle chain ring
 

So lately I have had problems with my chain coming off the middle chain ring when I climbing a hill. It only seems to happen when I'm mountain biking and not when riding on the street. Some people have told me that the chain ring is mostly likely worn, but it has under 300 miles on it. So I guess my question is what other causes could there be. I'm riding with a Sram cassette, KMC chain, and a Truvativ Firex Crankset. Any help would be great.

First of all is a rear suspension bike, or hardtail? I ask because I'm looking for something that would act differently on and off road.

If you watch the action of a chain feeding onto a chainring, you see that the points of the teeth must slip between the plates of the inner chain link. if the chain is feeding from an angle it's possible for the point to hit a plate and lift it rather than slipping in. If that happens the chain will derail.

The middle ring of a triple has shift gates to aid shifting. These also act as convenient places for the chain to derail if coming from an angle.

Take some time to pin down exactly when the problem occurs, for example only on the innermost rear sprocket, or with various ones? Are bumps a factor? Does it tend to happen at a certain point in the crank rotation? etc. As you pin down the variables, it might point you toward the source of the problem.

I asked about suspension, because sudden swing-arm movement can momentarily slacken the upper chain loop making it easier to drop on a hard bump.

I would suspect that your cables have stretched assuming that when you say the crank has under 300 miles that the bike does too. This would allow the derailer to move inboard and, while under torque and/or bouncing around during mountain biking, the frame flex would be enough to knock the chain off the middle ring.
When riding on roads or hard surfaces, you don't put as much torque on the frame nor do you bounce the chain around as much.