Hello Avinash
What happens is that the planned order start time is being calculated in the past. If the start in past is not allowed in customizing, system automatically changes to forward scheduling and uses today as the start date.
A shorter lead-time would solve this issue. If your material is procured externally, check if the planned delivery is not too long. If the material is produced in-house, check if the in-house production time is not too long.
Another possible solution is to allow the start in the past for planned orders. In this case, you should check the customizing on transaction OPPQ.
See the following link where the scheduling of planned orders is described in more detail:
Scheduling - Material Requirements Planning (PP-MRP) - SAP Library
BR
Caetano