MRP generated schedule lines not in sequence

Dear friends,

Client uses MRP to generate schedule lines for scheduling agreement.

For some materials, the MRP generated schedule lines are not in sequence (i.e not in ascending order of schedule line numbers).

Please see image.

For eg. schedule line 1 is followed by line 9 and then line 2 etc.

What could be the issue?

Please advise.

Regards

Prabhu