Agree, since it is a US destination, very likely to be the SSSS check, which seems to be becoming much more prevalent. I think c-w-s mentioned elsewhere they had been increased significantly as part of the deal between the UK and US on security issues like laptops etc.

The BP will be issued at the airport, and since you have an allocated seat and checked in, I'd concur - maybe arrive at airport a little earlier, but no cause for concern.