Because the squirrel you ran over is trapped in the mudguard!...No seriously though, could be any number of things but a leaking shock absorber will squeak and fail MOT. Simple shock test, push down on the car on each corner, it should rise back instantly, if not, have a look at the shock for signs of oil coming out of it and check the springs bushes and placement in case they've moved. Rusty springs will also creak. If you find a leaky shock or knackered spring, change both sides as you would brakes to keep balance in the car. Can throw tracking out if really bad too so watch tyres for wear on the edges.