This is pretty logical. MtGox halted BTC withdrawals because of the transaction malleability issue (which, due to MtGox's poorly made software, let people get their withdrawal payments more than once, leaving MtGox open to get its wallet emptied).
So now many people are stuck with MtGoxBTC, which is simply a balance in MtGox's database. MtGoxBTC is being traded for actual BTC (on exchanges like https://www.bitcoinbuilder.com/). It has a lower value since people are unsure whether or not MtGox will fix their system and let them take their money (IMO they probably will). Sellers are getting out to avoid the risk, and buyers are getting discounted (but potentially risky) BTC.
So now many people are stuck with MtGoxBTC, which is simply a balance in MtGox's database. MtGoxBTC is being traded for actual BTC (on exchanges like https://www.bitcoinbuilder.com/). It has a lower value since people are unsure whether or not MtGox will fix their system and let them take their money (IMO they probably will). Sellers are getting out to avoid the risk, and buyers are getting discounted (but potentially risky) BTC.