I was in the same situation when I dropped Enchanting and took up Mining at lvl 50. I actually didn’t encounter too many lowbies going for the same veins, but when I did I tried to be polite and let them have the node if they were clearly going for it before me. Also, if I found a lowbie mining in an area I’d often just head for a different area so that we weren’t in each other’s way.
Once I got up to Mithril and Thorium, though, the other miners were close to my level so it wasn’t such an issue. I had one incident in Un’Goro for which I got razzed by my guildmates. I (my female human pally, that is) was heading for a thorium node and just as I got within a few yards of it a female blood elf paladin of the same level arrived from a different direction. We both stopped short at precisely the same time, like we were both thinking the same thing: “Who was here first?”. I went ahead and took a step back to indicate, “Go ahead, you can have it.” When I mentioned it in guild chat somebody said, “Noooo! Don’t let the enemy have resources!” So I just explained I was hoping that by letting the belf have it maybe I could get some hot interfaction girl/girl paladin-on-paladin action 
The only time I actually “stole” a node was unintentional. I was in the Burning Steppes and spotted a thorium node on a small hill near a road. I heard the “clink” of a mining pick an instant before my own, followed by the sounds of combat from the other side of the hill. It was too late to stop my own mining action, and then a dwarf came popping up from the other side of the hill just as the node was fading out. Apparently the dwarf got jumped by a mob and interrupted just as he started mining, and due to his short stature I didn’t even see him behind the low hill.
A couple days ago I was farming iron in Arathi Highlands with my human DK. I was in the ogre cave in the SE corner and found a draenei fighting an ogre right next to two active nodes - 1 iron and 1 gold. So I stood back and waited to see what the draenei would do. She finished killing the ogre, turned and saw me waiting, figured out what I was waiting for, and said, “Go ahead, I’m not high enough yet.”

