There are few reasons that a scheduled order could be 
backordered during pick release process.

1. In-sufficient onhand
2. Order might be put as 'Pick release hold'
3. If it's a lot controlled item being involved then some 
of the factors will be checking at picking rule setup (like 
Lot- expiration date, etc) and tured to 'Backordered' 
incase the criteria does not meet.