Hardwood tree species under severe stress (drought & heat) or trees injured by fire or mechanical means "may" send up basal sprouts, typically from the root collar. Certain species are much more prolific in their "sprouting" abilities.
Are these basal sprouts stealing all the water and nutrients? Well, not really since the roots of the sprouts and the roots of the main stem come from the same tree. However, the allocation of the water and nutrients to support these sprouts is coming from the same root system that supports the main stem. I think it's more correct to say that the tree is merely reallocating its resources to maintain these sprouts.