You would get the same effect if you were towing a car, then suddenly jammed on your brakes. The car you were towing would, in all likelihood, collide with you. Would you blame the bloke in the car that you were towing, or yourself for failing to take account of the situation correctly?
Has there not been a report that the tug got a rope round her screw? At which point she would be unable to manoeuvre if the boat swung towards her and Astute may not have had enough room or time to stop the swing. This suggests itself to me because the rope must have been slack meaning the boat had come free and would have been swinging towards the vessel that pulled her off. I'm sure a BoI will sort it out. Meanwhile the pongoes can go on giggling, does no harm.
The main constructive point in all this is that a shoestring Navy predicated on each vessel being available whenever wanted will leave the Govt short of options whenever anything like this happens. We had a lucky escape when our SSBN bumped a Frog one (or vice versa) (and so did Fr.). Similarly with Endurance's flood off Chile. And these little happenstances are a normal part of seafaring life. See all the back numbers of 'Collisions and Groundings' for those who have access.
Way back Diamond collided with Swiftsure (senior officer). Swiftsure: "What are you going to do now?" Diamond "Buy a farm."
The tug's owner may have something to say about that. The Maritime and Coastguard Agency don't own the tugs and merely charter them, crew and all despite the paint job. IIRC at least part of the contract is coming up for renewal, or not.
Has there not been a report that the tug got a rope round her screw? At which point she would be unable to manoeuvre if the boat swung towards her and Astute may not have had enough room or time to stop the swing.
Rather unlikely as said tug has twin screws somewhat protected by Kort nozzles and she has thrusters at both ends. Mr Magoo would be able to keep that in a straight line even if one screw managed to get fouled.