Your cabinet looks to serve a lot of lines, so demand could well be high.
When BT install the FTTC cabinet, they don't fully provision it. As take-up increases, they need to add more line cards into the cabinet, and install more copper tie pairs between the PCP and the new FTTC cabinets.
Each time the cabinet fills up, it will become temporarily unavailable until the missing resource gets added.
Once your order gets accepted, you are earmarked for one of the available ports, and using available tie-pairs. Those should not be able to be over-booked, so your order should still go ahead.
It suggests something funny is going on here.