The fruit very likely has sugar, so it will cause fermentation to kick-off again when added, thus producing CO2. Obviously not with the same vigour as primary fermentation - although if you added a lot of fruit, maybe. Is the yeast able to get into the fruit? The skins are fairly tough for yeast, so will need to be broken somewhat. I typically mush my fruit a little with a potato masher (or slice - depends on the type of fruit).
I wouldn't be too concerned about the airlock not bubbling - this is mostly an indication that it's not perfectly sealed somehow. Or maybe it's going slowly, and you've not witnessed the odd bubble.
One other thing I would like to add - when I've added fruit to beers, sometimes it doesn't taste so good until it's been conditioning for a while. For example: A friend made a raspberry stout, and it was OK after a month or so, but after 3 months it was very nice indeed. So if at first the flavour is not pleasing, just wait.
Cherry dubbel sounds like a good mix BTW.