It's a little difficult to visualise RAT 5 unless you know the airport
As SLF at this airport and someone who has parked his car in the Long Term car park behind the aircraft tail a number of times, as far as I can make out, the aircraft did not move between
the photo in NutLoose's post #2 and
Fargoo's video snapshot in post #26.
That means the swirls you can see in Fargoo's pic are on the taxiway which runs parallel to the runway which is approximately 150m from the runway.
It is possible of course that having reached the taxiway, then being faced with whatever weather effect they reckoned out of their windscreen at the time, the crew may have decided a return to the runway was a more reliable route to the terminal. Although they were right on the tail of FR5172 coming in, no-one was on theirs. The airport is relatively quiet so a backtrack request would have been a fair shout.
Fact is though, whether they were intending to return to the runway or not, they sadly ended up off piste - Denmark is very flat (highest mountain no more than about 150m!) - but very few airports anywhere are wall to wall seamless smooth concrete every way you turn.
If they wanted to do it, the crew would have had to stop to liase with the tower before attempting such, having already vacated once after landing!
Maybe they intended turned 180 in what they thought was all still part of the exitway, and intentionally stopped deliberately abeam the 'M' boards before calling Tower, but they were by then on the wrong (western) side of the boards ... and er ... stuck!
Maybe ... all these maybes! ... it's definitely all a bit odd.
738 FR5172 landed just two minutes ahead and exited at the same spot and used the same taxiway, apparently without incident.