That's it, right here.  The block only permits a limited amount of zero fee transactions to be included, and any additional transactions must meet the criteria for the fee schedule to be included into a block.  Basicly, too many people are choosing cost of transaction over time to confirmation.  This is to be expected as the number of transactions increase, as this sets up a market for the fees (higher fees are more likely to be favored into the next block) while still providing for a free (or out of band transaction fee) methodology.