The only possible explanation I can think of is excessive and aggressive use of the brakes to keep straight resulting in a nose over. Doesn't apply to Cubs with bag brakes though!
I think the problem may be in the initial communication with the ground?
Correct me if I'm wrong, I only fly a small TD, but most large TD's need to be wheeled on and not landed in the three point possition- crap elevator behind big wing added to wing down app. for x wind add brakes and OOPs!!
Location: Quite near 'An aerodrome somewhere in England'
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Think gyroscopic effect! Yaw applied on touchdown could translate to a pitching force; depending on the direction of propeller rotation and direction of yaw this could be in a nose-down sense.
I've only seen this once when some idiot turned a Chipmunk rapidly with a large burst of power; this caused the tail to rise and a stiff breeze did the rest! This was whilst parking....
Methinks you're onto something. The gyro is a possiblity. Just to think aloud: If I need to yaw right to adjust a crabbed approach prior to landing then the force would be on a right turning prop (as seen from the cockpit as most are in 0 ATPL land anyway) would be precessed 90deg clockwise and result in a nose down. It is a little tenuous as the prop will be idling by that time and so the gyro effect will be minima, but it fits otherwise.
Beagle - this is not a criticism or anything other than awe. You seem to have such a vast experience and knowledge please enlighten us with a BRIEF resume of your past life.
"What are one of..." Should read "What is (the main) problem..." for an answer in the singular or "What are the main problems..." for a multiple. Better still, recast the whole question.
Re. Chipmunk turning. In days of old it was an accepted technique to give a burst of power with a little forward stick to lift the tail to make a tight turn possible. It is a tad hard on the tailpost, though, and thus rarely seen these days (when you are paying for your own maintenance). Some types require a little help to get the tailwheel out of the steering range to caster fully for a tight turn though this is usually accomplished with a touch of one brake.
I've never worried about nosing over on landing a taildragger; there are other things to think about at that time. Rough or waterlogged ground is more likely to cause concern.
I reckon the most relevant consideration is directional control.
The sitting and passing of ATPL exams requires, for the most part, the ability to read broken English, deconstruct and then reconstruct into the sentence that you think the examiner intended to ask!
I have sat and fully stared at a question for ten minutes before realising the best technique was to read all the available answers and then work backwards.
englishal, easier or harder than what? Do you mean is it easier to land a twin TD than a single TD?.. or harder? If that is your question, I can say it depends on the plane, but generally, a twin has more mass and is less likely to be squirreled about by cross-winds, however, a strong steady cross-wind has more push on the larger rudder area, and requires much more aggressive rudder inputs, including muscle strength. My first flights in a DC-3 had my legs literally trembling from hard thigh-muscle exercise.
But more to the point of conversation on this thread, gyroscopic precession does have a measureable affect on landing. A single-engine, when rotating the pitch for the flare, in very calm air, will yaw to the right during the flare. A twin with counter-rotating props will not yaw. A twin with both props rotating clockwise will yaw right during the flare, but not as much, it seems, maybe because of mass of weight, but it is there.
However, I don't think that there would be very much yaw on the ground after touch-down to cause a nose-over. All of these things mentioned lend to the tendency to nose-over. Also, is the basic fact that there isn't a nosewheel to prevent the occurance when to much brake is applied to stop groundlooping, which is the primary cause, ...I think.
High Wing Drifter. With about a thousand hours of tailwheel time and about 600 instructing on them, I can't think of any reason why landing with a crosswind should induce a nose over, unless you pork it up so badly you apply the brakes in panic. I suggest you accept the answer and tick the relevant box should the question appear in your P of F paper.
Working Hard - A resume of BEagle's career won't be brief. He was once seen swinging the prop at Kittyhawk!
I think the problem may be in the initial communication with the ground? 
