No one seems to have mentioned this bit.
On a nosewheel aircraft the c of g is in front of the mainwheels, in a tailwheel it's behind (otherwise they'd both tip over).
So you land a tricycle and you have drag acting behind the c of g. The resulting couple (remember your physics) tends to keep the aircraft in a straight line.
In a tailwheel you have the drag taking place in front of the c of g, so get the slightest bit out of line and you have a couple tending to yaw the aircraft. Let it go too far and the back wants to overtake the front.
Braking increases the couple and hence the effect.
And of course as the direction the wheels are pointing in deviates from the direction the aircraft is actually going in, it introduces a sideways force on the mainwheels, which increases the turning moment. Higher friction surface, more sideways force, so it's likely to be worse on tarmac than on grass.
That said, it's not difficult, just different.
Mike