Interesting.
In Gussman v Gratton-Storey the Defendant applied her breaks violently in order to avoid hitting a pheasant running across the road. The driver behind was unable to stop and collided with the Defendant’s vehicle. The Defendant was held liable so in this case the sudden stop was in effect held to be unreasonable. The lead driver was held liable in this case.
Putting aside the issue of a legal website that talks of 'applying breaks', this does clearly suggest that my barrackroom lawyer response is simply wrong. Looks, indeed, like OTH could be right, and the lorry driver is to blame for both crash 1 and the followup crash 2.....or maybe not. The gist seems to be, if driver 2 rear-ends driver 1, driver 2 is at fault, unless driver 1 stopped 'unreasonably', in which case driver 1 may be to blame. Is it 'reasonable' to stop suddenly just because you've crashed into a lorry?