If an account has too many open orders with a gross value of less than 0.0001 XBT, the account will be labelled as a spam account and all proceeding orders that are smaller than 0.0001 XBT in size will automatically become hidden orders.
Spam accounts are re-evaluated every 24 hours and may return to normal provided that trading behaviour has changed.
For more details on the spam mechanism please see our REST API docs on Minimum Order Size