Ensure Mongo sharding rebalance latest chuck
Yes, it is balancing "old" data because your choose bad shard key. Shard key should not start with linearly increasing key like time/date/number. Because (of the key) you add always data to the "end" of chunk, chunk is splitted at middle and lower (older) part is moved away. 

If you want to distribute your query/insert load evenly to whole shard you could use {pdm:"hashed"} as shard key. Of course changing order of those two keys {hdocid:1, pdm:1} would distribute load too.