Apparently they made the SafeCoin IPO (safecoin being the altcoin/blockchain that powers MaidSafe) available to purchase with both MasterCoin and Bitcoin. Based on exchange rates at the time, it was possible to get more SafeCoins if you purchased with MasterCoin, than if you used Bitcoin (since MasterCoin was so cheap).
Some people thought they could take advantage of the situation, and bought up MasterCoin to purchase SafeCoin. However it sounded like the IPO purchase cap for MasterCoin purchases was filled quickly, meaning a lot of people were left holding near-worthless MasterCoin. Thus began the cries of "Mastercoin pump and dump", which the IPO had (inadvertently?) caused.
Some people thought they could take advantage of the situation, and bought up MasterCoin to purchase SafeCoin. However it sounded like the IPO purchase cap for MasterCoin purchases was filled quickly, meaning a lot of people were left holding near-worthless MasterCoin. Thus began the cries of "Mastercoin pump and dump", which the IPO had (inadvertently?) caused.