It's the nature of the beast. Most of the Asian hybrids produce epicormic growth as a consequence of their growth habit. Most of my mags have never been pruned, other than removal of dead growth, and most of them produce sprouts like you've shown. The older the tree, the more sprouts. Mostly I let them be, but if they die, and many do due to lack of light, I remove them. It has been my observation that southern mags and the native big leafs also possess this trait, but to a lesser extent. If they bother you remove them, no harm done. I'm not aware of anything you can do to prevent them.

This is one of those tree species where excessive pruning should be avoided as it will generate water sprouts/epicormic growth. "Heavy pruning of mature magnolias can induce watershoots (long, vertical, vigorous shoots) and dieback. Routine pruning is therefore usually restricted to removing deadwood and watershoots" RHS

Dowoods are another species that react poorly to heavy pruning.

It's trying to replace the branches you cut off. As with any kind of tree numerous water sprouts coming from the sites of branch removals after pruning = too much pruning. Since presumably the original root system is still present, in its previous size the tree will continue to try and restore the total volume of top it had before. In addition to top pruning Bonsai practitioners use annual root pruning to reduce new top growth. But duplicating this technique is not feasible in your situation, with a subject of this size and nature. One that is also growing in the ground.

Deciduous magnolias have a more vegetative phase where development of crown size dominates and then a flowering stage where they become twiggy and develop large numbers of flowers. So I would say they become less apt to sprout with age, rather than more. And saucer magnolias may grow something like 50 ft. across with age so yours does not appear "very large" in your picture to me, may still be in a relatively youthful state.

what if he removed it back to the trunk???

im not sure i would have made a cut on that large a branch.. that far from the trunk ...

ken

It looks like the branch that was cut was interfering with the house.

Mike the original post says

>It needed quite a bit of pruning when I moved in ~15 years ago. I'm satisfied with the shape now, but at a few locations where I pruned off medium size branches, I now get recurring suckers<

And I see a section of remaining twigs to the right of the circled water-sprout, closer to the house. Otherwise, as far as can be seen in the shot provided the specimen appears to have been thinned to produce a structure consisting of patches of normal, twiggy growth with unnaturally large gaps between. With the water-sprouts occurring where the tree is trying to fill these gaps back in.