I think you hit the nail on the head with lifting too heavy and sugar. It does you no good to have muscle failure before your done with your sets. And along with sugar it's the same with certain carbs. I try and stay away from certain carbs like bread and potatoes. But when I do carb cycle on the workout days on oats and sweet potatoes I still try to go easy on it so I don't go overboard and kick myself out of ketosis too much.