When you accept a bid on a player via the list of incoming bids, an auction will be started. If there are other bids on the same player, with the same or a shorter duration, these will also be activated.
Example:
Club A has made bid of 50.000 Credits with 100.000 Credits as auto bid. Club B has made a bid of 20.000 Credits with 150.0000 Credits as auto bid. You (the seller) accept Club A's bid. Now, Club B will be the high bidder with a bid of 100.001 Credits. This is because Club B's auto bid was higher than Club A's.