I agree with what John said.. use the strands from the tip of the bucktail first, these are generally the thinnest and softest. If you look at the bucktail you will see that as you move towards the base, the hairs get thicker. The hairs at the bottom are the ones that will most likely splay when you tie them in.
If you are still having problems, try adjusting the pressure you use on the tying thread. Make a few wraps with even pressure, then gradually increase it once the hair has been secured. You may also want to try taking a loop of thread around the bucktail completely at first, without going around the hook shank. Then make your next wrap around the bunch of bucktail and the hookshank, holding it in place. This tends to keep the hairs together and distributes the force of the thread evenly.
If none of this works, you may want to try another bucktail. Its hard to pick a good one until you know what you are looking for. I have at least one of every color that is sitting here unused because the hair isn't good for tying, or I have already taken the small usable portion.
Email me if you have any other questions, or post them here