Strange take off

[brums evil twin]

Flew out to Belgrade a week or so ago, with air Serbia from Heathrow.

Pulled on to the end of the runway, engines up, brakes off, accelerate then a little way along the runway at a decent lick of speed, brakes hit very hard, hear tyres lock, then brakes off and all the gas on the engines to now take off with a lot less of the runway available.

So no explanation from the captain once in the air.

So why?

I guess something not cleared the runway, but why start in the first place? Big bird flying across? Warning light?

Any way, we got up and away

[Boosted LS1]

Probably brake testing the plane behind.