Okay, I understand that difficulty has gone up a lot since June 12, so there is a lot more competition, but I still can't see any mathematical reason for the share price of any security to go down exponentially when BTC price is going up.
Because you bought a fixed MH bond, aka a turd. As others pointed out, your "dividend" is linked only to difficulty and block reward. Difficulty was always expected to go up, perhaps in the past months it went up faster than most expected, but with ASICs approaching, you havent seen anything yet. I fully expect difficulty to go up >10x in the next 6 months and possibly as much as 100x in the next year. On top of that, blockreward will halve in december, and thus so will your dividends. Well, it will be half whatever is left by then, because by december ASICs will have started mining. Gigamining offers you a free upgrade when they get their asics, but a 4x increase (IIRC?) will do precious little to offset the difficulty explosion you are about to see.
TL'DR, these mining bonds have been overbought since day one by people who didnt understand what they were buying. Im pretty sure they are still grossly overpriced today, so anyone who believes otherwise, lend me your bond, so I can short it. I will give you a nice bonus on your dividends if you let me borrow it for 3-6 months. PM me for details.