I think the answer is in the question really. Bipolarity, in its essence, is bouncing from the extreme highs and lows without a stable middle ground. Bipolar people can be very emotionally turbulent and can change their outlook on things quite regularly.

As someone who has lived through some very poor states of mental health, I can tell you that it's quite common for depressed people to push others away because they either don't feel worthy of the attention or they don't feel as though they can't face you seeing them at their lower points.

It's a tough position for you to be in, as you can only be there for them and try not to take it to heart too much if they push you away at times. In a warped way, they're most likely thinking that they're doing what's best for you, rather than themselves. That's not to say you have to put up with it though, or that you should be the person that bares the brunt of the other person's issues. You can only help someone who wants to be helped.

I'm assuming a lot of stuff here, and I don't know enough about your relationship to say anything for certain, but I hope what I've said might be useful in some way.