It actually happened in 1981 as well – I have Automobile Year 1981 at home and it shows as much. I believe it was for safety purposes; if the race had started from where the finish line was located, the cars would all be going into a tight sequence of bends almost straight away. But on the other side of the circuit, there was a fast sweeper and a long run down to the hairpin, giving the drivers time to sort themselves out. It wasn’t the finish line that was moved, but the start line, if that makes sense. The extra half a lap came at the start of the race, not at the end.