Volos/Nea Anchialos can also have some dirty winds off the hills. The runway is long and smooth there, with the exit to the civil apron at the end if landing from the sea - no need for heavy braking.
To be honest, a 4.5 is not really that bad - I've witnessed a lot worse. However, I seriously doubt that the aircraft was "pitching and rolling" on the runway.
If arriving from SKP (Skopelos) they route you to the AGH VOR which is on the field or you go for a straight in ILS - if you get a straight in, you are invariably always initially high on the approach profile due to the sector controllers not giving you a lower altitude initially. Volos must have some of the worst ATC in Europe.
I'd go with the thermals theory with a bit of dirty wind on the approach.