Have dm type 2. Ate high carb meal, and 6 hrs later sugar was 68. Why would this suddenly happen?

Normal. If you're on medication, it could be explained by the action of the medication. If you're not on medication, what happened is that your sugar when way up with your meal, and then your body had to put out a large amount of Insulin to bring it down again. A glucose of 68, particularly if it's a finger stick not a lab glucose, is normal. Nothing to worry about, but eat less and lose weight.