What does it mean when pine needles turn brown on the tip?

I was born with a brown thumb but recently I have become fascinated with bonsai. I bought a book which has been very helpful getting started, but it doesn't tell me everything. A few weeks ago I went out in the mountains and found a few small pines that were already somewhat naturally stunted from growing in rocky soil. I dug them up and potted them and they seemed to be doing well, untill a few days ago some of the tips of some of the needles began to turn brown. I think I have possibly been overwatering them, but is there anything else that would cause this that I may be doing wrong, perhaps too much/not enough sun?