Reoccurring confirmed gas issues are often going to be caused by food or sometimes stress. Intestinal blockages from something ingested or a problem in the intestine can also be a cause, though this will usually be a more acute problem and not reoccurring. And when this happens it's usually much more serious and may require surgery to correct.
With diet it could be sudden food changes/introduction(pellets, veg, greens, fruit, grains/carbs, etc) altering the delicate microbial balance in the gut, too many high carb/sugary foods(which can include pellets, particularly certain brands) altering the bacterial balance in the gut, sensitivity to a new food, veggies/greens/fruit that are off/started to spoil, packaged herbs/veg that is treated with a wash to prolong shelf life, or sometimes a newly opened bag of feed or bale of hay can be tainted or bad in some way with mold, pesticides, herbicides, or tainted ingredients for pellets during the pelleting process. If you've recently opened up a new bag or bale just prior to this starting, I would be looking at that as the possible cause.
Then there is stress that can start to alter the microbial balance in the gut. Changes in the environment; new pet or person in the home, furniture changed around, environmental noises, changes in temperature, change of area, new rabbit moved next to their area. Anything new or different has the potential to be a stressor.
Though I would say if the gas issues are happening in multiple rabbits that previously never had a history of it, then a food issue is the most likely culprit.
