Are they both neutered? Since it's spring there may be a female dog in the neighborhood that's in heat and triggering their hormones. Another possibility is that one of them has a full anal gland. This will often confuse the issue too!

You might try seperating them for awhile. If they whine and and make a fuss while trying to get back together; you can be pretty doggoned sure it's either hormones or the anal gland.

If it's dominance, (which I doubt) they'll work it out before too long.