I imagine that in a captive 'Safari Park' situation, a group of lions would attack anything that triggers their hunting reflexes- so a tractor would stimulate that reaction. Because their hunting instincts are not utilised very much in captivity, I guess they would go for anything, be it a tractor- or a hyaena.
I've never been to West Midlands or Knowsley Safari Parks.(Longleat I have been to a number of times but not for many years). They both seem to have interesting collections though. After visiting Woburn, I'm now more interesting in seeing the other Safari Parks too(!!) so maybe I will do so in the coming months.