I have never seen such high interest rates for bitcoins on Poloniex.com before.
Lately the rates are between 36.5% to 109.5%.
Probably it is related to the likelihood of a bitcoin hardfork. Poloniex has announced their position on the possible hardfork: https://poloniex.com/press-releases/2017.03.17-Hard-Fork/
"What if I have my Bitcoin on loan at the time of the fork?
If you have Bitcoin on loan at the time of the fork, you do not have it on balance and therefore you cannot receive corresponding Bitcoin Unlimited."
Very analysis you have done. Great and informative to see deeper observation made by you.
I assume you get interest by lending your BTC so that other people can short it. A higher interest rate may mean that more people want to short BTC.
This can be due to either a bearish view of the currency or just a larger market. A larger market will lead to more players and therefore more shorts.
Great work to watch out for