PP - I think that too long a fork as well as creating the slow handling you mention, can also lead to understeer as described.
Yeah, that occoured to me after I posted too, but I've not had the time to edit or comment since then.
You cannot get understeer on a two wheeler
Sorry TJ, you usually seem to know your stuff but on this occasion you're 100% wrong.
Definition of understeer
Understeer is a term for a handling condition in which during cornering the circular path of the vehicle's motion is of a greater radius than the circle indicated by the direction its wheels are pointed. The effect is opposite to that of oversteer. In simpler words understeer is the condition in which the vehicle does not follow the trajectory the driver is trying to impose while taking the corner because the effective slip angle at the front is larger than that at the rear, instead following a less curved trajectory
So, if you're front tyre starts to slide before the rear, that's understeer. I'd imagine you've had it happen. I imagine every MTBer in the world has at some point 'lost the front end' at some point. The problem being when this happens too readily, at too slow a speed. Then something is wrong and you have to do something about it, as our OP has found.