The 11,250 figure is based on the losses and how much was taken from your lending balance. The actual figure could be higher, but probably not lower, depending on mainly if they covered some of the losses.

On May 26, Poloniex suffered major losses. On June 14, Poloniex socialized these losses on lenders of BTC at the time. I can't say with authority why there was the long delay, or what happened in the meantime. I would speculate on May 26, Poloniex took over the loan clam positions from those with negative equity and held the position for three weeks. I think they were hoping the price would increase enough so they could offload the position either over time, or in an OTC trade at a high enough price so all loans could be repaid. Poloniex could have continued renewing BTC based loans to cover the short BTC position, and so lenders would not start withdrawing to other lending platforms. Once they saw the price of clam was not going to quickly go back up, they imposed losses on btc lenders.

If you did not have a BTC loan open as of May 26, you may have a valid cause of action. The reasoning for this is simple, you should not have to cover losses for loans that you did not have open at the time of the loss.

If some lenders were able to withdraw their coin from poloniex, and the percentage of your coins taken to cover the losses was higher because of this, you may have a valid cause of action for only your additional losses. This would not apply if Poloniex seized other assets from these accounts that withdrew to cover their portion of the losses based on their btc loans outstanding as of May 26.