Was thinking more on the Oslo end, but if nobody else is flying to LYR, it wouldn't be that. Unless you're also flying TRF-LYR or something? Those two routes would compete with each other for some of the same demand.
Other guess is D-check for the plane flying meaning it doesn't get flown for a month and demand drops, and then comes back once it is flown.
There are no flights from TRF or RYG to LYR to interfere with OSL-LYR (by me or anyone else).
There are passenger flights from BGO and TRD to LYR, but they are far enough away from OSL that there should be no impact and as they are pax flights, the amount of cargo uplift is minimal, even if there was some overlap.
Interesting idea about D check (although I would think that maintenance wouldn't impact actual demand) but the last D check was almost three years ago, so that's not it either.
So, I think it is safe to say, something isn't working quite as it should
