I saw a few planes that appear to be flying as if they're travelling in a diagonal line to their heading.
As Rainboe says relative motion but also can be an illusion. South af Ayr is a place known as Electric Brae, if you go there the illusion is basically it looks as though you are going uphill when in fact you are going down hill!
Also can be due to looking at fixed reference points. Saw a Tornado on Monday low flying in the distance. it looked as though it was in level flight at an extreme 30 - 40 degree nose up attiude, the second a/c of the pair appeared level. Realised it was my brain being tricked by the reference points is was comparing it to!