Purchase order changes that requires reapproval

hi,
I tried to update the business category of an approved Purchase order and the status
was change to need re-approval. Being the buyer, i click the "Approve" button.
Per action history there was an additional event of my resubmission and approval.
In the printed Purchase order, it reflects that i was the buyer and the approver at the same time.

also kindly help me what are the events that will trigger purchase order to be needs reapproval.
will it go to the complete process of approval?

thanks,