One thing that it could be is residual salt buildup in the coco. Since coconuts are processes on islands in the middle of the ocean, they use saltwater to wash and process. Higher quality coco mixes will do multiple fresh rinses to remove the salts, but if you don't give the stuff a good soaking before potting up with it, you can get nute lock-up and salt burns as the root tips hit the harsh coco.
It could also be due to pH fluctuation, but normally a drastic pH swing will manifest as blotches of dead tissue on the leaves. It sounds like you may have a problem with salt buildup in the medium, since it's just the tips being affected. Check out the coco grower's thread for more info.
