I think it's tricky for beginners because they have probably never flown anything quite like it...
...unless they have glider time!!

Except for the doubled-up main wheel bogey, that configuration is just like most gliders, and the long wing makes it similar, too.
Comes back to what I've always said. All pilots should have to take a gliding course before being let loose on powered aircraft.
It seems very odd to me that with all that wing area & flaps like barn doors the U-2 appears to have nothing in the way of lift spoiling devices.
The glider I fly has outrigger wheels on the wings and it normally comes to rest on the skid forward of the main wheel as it slows down, but it's still possible to ground-loop it. Many of the faster gliders are quite easy to ground-loop.
I've spent a lot of time around gliders & I've never seen one ground loop. This might be because they're usually operating from grass fields & landing directly into wind.