All of my un-scientific reasoning, based upon my sole observations, suggests a very active root system. Suckering can occur also in the form of "shoots" from the trunk. It was my understanding that a sucker was a shoot and not a stem, hence sucker. I'm still waiting to see how my willow sucker transplants "whips" will do this season.

Here's a little reading, it seems to have a few ideas:

http://www.sacbee.com/static/archive/home/gardening/garden_detective/2001/0908a.html

My take: Many woody plant species have dormant buds in stem and or root tissues, at or below soil surface. Frequently, the highest density of these buds is found at the root collar (or basal flare), but many species contain these buds in roots near the surface.

In a normally growing tree, the hormones auxin and cytokinin have something of an antagonistic relationship, with the auxin stimulating apical growth and suppressing cytokinin's stimulation of lateral or secondary (to apical) meristems. Without getting too complicated here by getting into branching index, it should suffice to say that anything suppressing the flow of auxin will normally stimulate dormant buds. Since auxin is produced in apical meristems and leaves, tree injury (including pruning) that reduces auxin production and its suppression of dormant buds allows cytokinin induced activation of the buds.

Of course, we would look first to pruning, especially improper (as in tip pruning) or heavy pruning as causal. A few other causes might be insects, wind damage, disease pathogens, cold injury, etc.

Al