As I think someone touched on 'upthread', a major consideration is also overcrowding that can occur on platforms. Take Surbiton, and yes I'm talking historically - pre covid, but terminate one or two UP peak hour 10 cars there, combined with the usual commuters, and p1 & 2 can become horrendously overcrowded. So then you have following trains stacking up outside the station and a very real risk of passenger DIY egress can become a possibility, which is why (IIRC?) in such instances, instructions were issued some years ago whereby trains should only be held at platforms. Overall, it can be a prudent measure to 'stop-all' initially, and then get things moving in unaffected areas as and when the crewing side of things is sorted, and by that I mean individual crews have to be contacted and agreements reached, bearing in mind most crews in the early a.m. will be sooner or later expecting to work in to (in this case) Waterloo.