Place the garlic, oregano leaves, cumin, salt, pepper, guajillo chiles, agave, pineapple juice, white vinegar, achiote paste and 1/2 cup water in a small pot. Bring to a simmer.
Cook for 3 minutes, then cool for 10 minutes.
Pour the marinade ingredients into a blender; blend until smooth.
Place the pork in a large bowl and add the marinade. Toss to coat completely. Chill, then refrigerate for at least one hour, or up to 24 hours.
Preheat a grill or indoor grill pan to medium high heat.
Place the pork and pineapple slices on the grill. Cook for 3-4 minutes per side, or until pork is just cooked through.
Coarsely chop the pork and pineapple.
Divide the pork between the tortillas. Top with the chopped pineapple, red onion and cilantro, then serve immediately.