I would say the reason was a ham light plane pilot rabbiting on and not letting business conduct itself.
Speculative b@llocks and arrogant too.
The light aircraft pilot may have been reading back a modified clearance or passing some other fully justified message.
Furthermore, s/he has equal right to use that airspace as a Ryanair 737.
I've had to go around in an SEP because of a similar transmission by CAT and I didn't throw a hissy fit like Rainboe, just accepted it as part of the game.
I didn't burn 800kgs of fuel, but in the other hand I did pay for the extra out of my own pocket, unlike the FR drivers.
What's good for the goose.....