In a small bowl, stir together the pineapple juice, teriyaki sauce, honey, and red pepper flakes, divide in half.
Preheat the grill or grill pan to 425°. Arrange the pork, pineapple, peppers, and onions onto the skewers and brush with the sauce. Brush grill with oil, place the skewers onto the grill, and cook about 15-20 minutes, turning as you go, until the pork is cooked through, basting with the sauce as it cooks.
Meanwhile, in a small pan over medium heat, add the remaining sauce and reduce by half.
Remove the skewers from the grill, glaze with reduced sauce, and allow to rest for 5 minutes before serving.