The 707 -which was a very modern plane back then- suffered what is known as a trim runaway, an unwanted continuous operation of the elevator trim, causing a slow but steady and ever increasing pitch variation of the plane (in this case: nose up).
The trim runaway started somewhere during the initial approach and the crew fought increasingly hard against it during their ILS approach, but in the end could no longer hold it and had to make a go-around on final. The plane initiated a turn and climbed steeply to around 1,500 feet before stalling and crashing to the ground, killing all 71 persons on board as well as 1 person on the ground.
A stewardess on board of that flight happened to be a niece of my grandmother, BTW.
For some time after the accident, Sabena -and I think many other airlines too- had a set of strong steel cables fitted to the overhead panel of the cockpit so that in case of a trim runaway, the trim wheel could be physically blocked by the flight engineer to prevent catastrophes like SN548 from happening again. Don't know if it would have worked though....
The subsequent investigation fully cleared the crew from any wrong-doing and concluded a structural error to be at the basis of this tragic loss. It made several recommendations to Boeing to improve the design and thus safety of the flight controls on their 707.
Until today, it is not known what exactly triggered to electro-motor to run continuously, but since then, trim mechanisms have been altered so that pilots can always either physically overpower the trim force or alternatively that the system disconnects if it feels any force opposing its operation.
This crash was the worst accident in Belgian aviation history and also the last fatal accident for Sabena. There is a small memorial in front of the catholic chapel of the old terminal in BRU
where you can see the names of those killed today, 45 years ago.