London Heathrow is one of the busiest airports in the world. Airlines jockey for position when and if any slots become available. In the past, airlines like Oman Air paid over $75 million for a slot purchased from Air France. So when London Heathrow announces that slots are available, all airlines listen.
At the end of last year, London Heathrow announced that 16 slots were being opened and that there would be a lottery to determine which airline would be awarded these highly desired positions for the 2020 summer season. Much to everyone's surprise, Norwegian Air was allocated six weekly slots (i.e. enough slots to accommodate 3 roundtrip service per week).
Norwegian Air has never operated out of London Heathrow and does not have the connectivity from that airport that it can benefit from compared to London Gatwick. This would be a new venture for Norwegian that had previously stated that it was focusing on profitability and not expansion at this time.
Now, after a few months of consideration, Norwegian Air has declined the slots and given them back to London Heathrow. The airline stated that after much review of their current schedules and the fact that the Boeing 737 MAX planes are still grounded, it does not believe that these slots would be in the carrier's best interest.
Norwegian Air cannot sell the slots because the laws state that airlines must operate the time slots for two years before they can sell them to another airline, so in essence, Norwegian Air is merely returning them to the airport.
London Heathrow has not stated who they will reassign the slots to as of this time. The airport has a tendency to offer new slots to airlines that either has a minimal presence or no presence at all at their airport before giving more slots to the larger, legacy airlines.