I think he's saying that the main event loop has called a callback, only to have that callback either call the main event loop, or something similar that proces

I think he's saying that the main event loop has called a callback, only to have that callback either call the main event loop, or something similar that processes events until a certain condition, and that in turn may cause other callbacks to be called. Stack depth has increased. And it tends to lead to bugs because sometimes unconsidered things happen in the midst of the first callback's processing.