Key points that started it were (you can see the chain of events in the doc):

2.4.1. At 16:52:33 on Friday 9 August 2019, a lightning strike caused a fault on the Eaton Socon – Wymondley 400kV line. This is not unusual and was rectified within 80 milliseconds (ms)

2.4.2. The fault affected the local distribution networks and approximately 150MW of distributed generation disconnected from the networks or ‘tripped off’ due to a safety mechanism known as vector shift protection

2.4.3. The voltage control system at the Hornsea 1 offshore wind farm did not respond to the impact of the fault on the transmission system as expected and became unstable. Hornsea 1 rapidly reduced its power generation or ‘deloaded’ from 799MW to 62MW (a reduction of 737MW).