Years ago I was taught that overly-heavy pruning, especially in spring or summer, encourages epicormic growth. That certainly appeared to be the case with an ornamental crabapple that was "pruned" locally, it responded by sending out a large number of watersprouts. From what I've read it likely has to do with changes in the level of light, such as from this type of heavy pruning, or a nearby tree falling down and letting in more light.
It looks like in the picture of the oak there is also a vertical crack in the trunk in the area of the watersprouts??
