I do not know the answer. I only think several possibilities.
As Curran said, it might be the annual sawing off of the deer's horns. The event begun in 1671 to protect both people and deers in Nara (奈良).
I also think other possibility. Horns of a deer come off naturally in every spring. Watching the first tsuba, the cut sections of the horns look as if they came off naturally to me. And there are some insects on the tsuba, though at first I thought they were birds from their faces. What kind of insects they are? I thought they are bees or winged ants, maybe bees. Then fell out horns suggest spring, and the season word for bees is also spring. That makes sence, I think.
Then why are there horns and bees? I found an old proverb that a bee stings the horn of a deer. The meaning of the proverb is that being indifferent whatever have be done.
IMHO.