I have no answers as to why your Glenn is doing that, but I'm in the same boat with my PPK. Every year the tips of branches swell with buds, and by the time I've lost all hope, it pushes its one flush a year. I thought the spot it was originally planted in was the problem, so relocated the tree, and it did the same thing in the new location. I planted a seedling in the original spot the PPK was in, and it, like all my other mango trees, had multiple flushes of new growth this past season.
Maybe we got bum rootstock or bum scions? Either way, it's frustrating.