There are rumours that this slot is the MIA-LHR slot that BA/AA were forced by the EU to make available for MIA-LHR nonstop service as a condition of their antitrust immunity application two years ago. DL grabbed the MIA slot (plus the two that BA/AA were forced to provide in BOS) and gave it a go. After driving down MIA-LHR fares for a year, DL gave up. DL has also given back one of the BOS slots as well.
Nope. AA isn't giving up anything. BA is providing these slots from its inventory and leasing them to US. Apparently, US petitioned the EU to allow it to fly a one-stop via CLT instead of the nonstop MIA-LHR as required in the ATI approval.
I don't think it's seasonal, but BA/AA don't have to provide the slots for more than 2 or 3 years, and that runs out sometime next year.
MAH4546 posted elsewhere that he thought that the approval from the EU requires same-plane service as a condition of the one-stop approval. If he's right. that will be one empty A330 between MIA and CLT every day. NRSA heaven.