Trim The Roast Of Any Excess Fat, And Cut Into Four Pieces. Rub With Black Pepper, And Place Into A Slow Cooker. Add The Onions, Green Chiles, Chili Powder, Cayenne Pepper, Garlic Powder, Cumin, Chipotle Peppers, And Beef Bouillon. Pour In Enough Water To Cover 1/3 Of The Roast. Cover, And Cook On High Until The Meat Begins To Fall Apart, About 8 Hours. Add Water If Needed To Keep The Roast From Drying Out. Turn The Slow Cooker To Low, And Remove Meat To A Large Bowl With A Slotted Spoon. Shred The Meat Into Bite-Sized Pieces Using Two Forks. Return The Meat To The Slow Cooker; Cook About 20 Minutes To Reheat Before Serving.