why does the orange juices bought in cartons sometimes taste bitter?

Answer 1

Try shaking it because the sweet part might have settled to the bottom. It's usually reconstituted from concentrate and the water can separate out somewhat. But it might also be more sour than other kinds because some are from cheaper and less sweet oranges. The best oranges are saved for selling to eat.

Answer 2

My guess would be that they don't use the best oranges possible. Could also be that they squeeze the crap out of them so they get more per orange, which means they would get some of the bitter rind juice in there.