Strange landslide in the UK

Farmers Weekly Interactive carries a rather strange report and some excellent photos of a landslide that occurred near to Widnes in NW England on 29th February:

"Tractor Driver James Fletcher who works for Chester-based contractor Mike Harley, had a lucky esacpe when the field he was ploughing gave way beneath him. He was on the final run of a sloping field near Alvaney, Widnes, when the bank he was on collapsed, sending an estimated 15,000 tonnes of soil sliding down the slope. Mr Fletcher's tractor and plough was carried a quarter of a mile, through 360 degrees, crossing two fields and ended up being half buried in soil. Amazingly, the tractor started again once dug out, but had to be taken away on a low-loader."

IC Cheshire Online carries a rather more detailed report:

A wave of mud and rocks carried a tractor through three fields with its driver trapped helplessly inside. James Fletcher was ploughing a potato field at Teuthill Farm, Alvanley, at around 12.30pm last Friday when the ground gave way around him. He was trapped at the wheel as his tractor was dragged on its side through three fields and only escaped when the tractor hit two trees, smashing the windows so he could jump free. James, 45, of Bates Lane, Helsby, said: “I got there at about 9am and this was my last run up the field. “The next thing I knew, I was still moving forward but I was going backwards as well. The tractor was still ploughing and I just thought I was getting stuck. “Then the ground opened up and spun the tractor around. It smashed me into an oak tree which knocked the tractor over onto its side.“I was trying to open the door but it was too high above me. I was frightened to death.“I hit another oak tree on the other side which righted the tractor.“At the last minute I saw the house and I thought: ‘It’s time to get off now’. “If I hadn’t got out that would have been it.”

From what one can work out, the key facts are:

1. The landslide was triggered by a tractor with a large plough;
2. About 15,000 tonnes of material failed, travelling for about 500 m down a fairly gentle slope;
3. The rate of motion was sufficiently fast to prevent the driver from escaping (the driver was very lucky by the way - if the tractor had rolled completely over, rather than righting itself, he might well have been killed.

This landslide is intriguing because of its exceptional mobility and speed of motion. In many ways it appears to be similar to a quick clay failure, but this is unlikely. Data from an amateur weather station at nearby Widnes suggests that there was not an exceptional of rainfall that day (2 mm), or indeed for the preceding few days. Thus, the nature and cause of this landslide is somewhat perplexing.

I am trying to find out more about it.