Openreach will only allow one ‘order’ on a line at any one time, be that a transfer, active line takeover or cease. When EE placed your order they’d have sent a letter to the current occupants saying they’d be taking over the line. The occupants then spoke to their ISP to say they didn’t want the line taken over and to move their service to the new address they’re moving to. It’s a pain and happens all the time.
Not sure details as my brother dealt with it but our buyer took over our old line (moved from BT to Now TV) on the day and we took over our seller's on the day (think they were on Talk Talk, we stuck with BT) and they transferred to BT at their new premise.