Could it be that passengers who are connecting into LH flights are being progressively put off transiting through LHR/LGW and increasingly STN due to overcrowding, terminal changes and slow security.
I booked a holiday today to Sint Maarten and had the choice of NCL/LGW/SXM using VS via Barbados or NCL/AMS/SXM using KL throughout. The price difference was negligible so its AMS for me. It is a much more pleasant airport to transit through and 1 carrier throughout appeals too.