That was a severe swing to the right, and an equally quick correction. I would support the theory that the 'straight track' is the starboard reverse, possibly at high power until ?reduced? to correct the swing, or did its trail fade as perhaps the wings levelled? Equally, as Rainboe says, it could have been something 'hot and heavy' and detached. So, was it asymmetric reverse or the right wingtip drag that caused that dramatic swerve to the right?
It is now possible to tie down the exit point from examination of the R18 touchdown markers, and it would appear to me that the initial touchdown was off the runway and already angled well to the right.
I have tried zooming to max pixellation and cannot identify the debris between the wheel tracks and the runway. Is that part of an engine casing just right of mid-picture?