High fiber content most likely. Fats (like oils used for cooking) and similar can also increase how fast things move through. Also just general different composition if you have a poor variety in your diet (like I do sometimes) can make for more surprises.

If you don’t normally eat a lot of fiber (like most people should do, especially old people), a sudden influx can really make for some interesting mornings, especially if there’s a lot for your body to clean out.

It could very well be clearing out some minor backup. You should analyze your diet for recommended amounts of fiber and other similar things and eat more of them, not less.

The poop doesn’t magically materialize. If you’re pooping more than you ate, you have backup, and should eat more things that keep your bowels going healthily.