Any Rut activity this time of year is very, very spotty. Any Buck that has hard antlers is capable of breeding, but it takes that stray doe coming into heat this time of year to spice things up for a bit.
I doubt Bucks spar for the sole reason of trying to knock their antlers off, but they could easily loose one this time of year while doing so.
When the testosterone level goes down it will cause a Buck to lose his Antlers. Physical condition, participation in the rut, etc. all play a part as to how fast that happens.
Dan