currently mongodb will rebalance chunks on inserts, but in some cases the collection key is a numeric sequence which makes all first X amount of records go to one server and the second to the other...
this is ok as long as the load on that collection is not high, but if the collection usage is high, it means that manual sharding should be invoked by the admin.

i think that re-balancing a collection based on its load should also be added to the balancer. when a collection chink have a high load of queries the chunk should be splited and move to the other server/servers...