Here's the thing . . . spoiler alert . . . stuff happens!! I don't know how many Greater Anglia RRB duties are operating today . . . it looks like 3-4 service driver duties are uncovered . . . one of the routes is a Sudbury local service, with 2 buses / hour cancelled, so that's probably 20% of the cancelled journeys accounted for. The Konect site notes all the cancelled trips (some of which are attributed to a "medical emergency", and others to "staffing complications"). . . I don't do twitter, so I don't know if these are shown there as well.
I will be the first to admit that this is a terrible day for them, but before condemning them outright . . . let's assume that all duties were covered on Friday evening. A driver is taken ill overnight, and goes to hospital; their partner is also a driver, and goes with them. Reasonable so far? Stuff happens, and sometimes there actually is no immediate solution . . . no extra drivers are available at short notice. Driver duties are long now, and staff may actually be "out of weekly driving hours".
To put this into perspective . . . a driver on a 4-day week may well work almost 10 driving hours each day; that's 40 hours. They may work an extra day; that's now 50 hours. The maximum in a week is 56 hours; 90 hours in a fortnight. A driver may wish to work, but can't!!
Let's now look at RRB coverage. Work will be (usually) allocated at the beginning of each week to each garage, and they will (normally) accept such work that they can expect to cover. At my company, the driver allocator at each garage would receive these details on a Monday, and would cover them as they could. By 1200 on Wednesday, they would confirm all duties were covered, and if they weren't covered hand them back for coverage by other garages.
These extra duties would now become the allocated work for the weekend. I don't know if standby drivers are available . . . I expect, at the smaller garages, probably not. So . . . a few drivers have become un-available, and there are no spare drivers available. What should the garage cut? All the RRB buses? That would seriously upset the rail passengers, wouldn't it? Cut the service journeys? That'll upset the regular passengers greatly, won't it? Damned if you do . . . damned if you don't.
I've been in this sort of situation (in London) . . . and being castigated from both sides isn't nice. In my case it was a flu epidemic that decimated the garage for four days over a weekend. Sometimes . . . stuff just happens. A little bit of understanding doesn't hurt. If it happens week on week . . . that's different.