1) Too cold for the yeast: Make sure it is warm enough for the yeast. Active ferment will warm beer up sustaining the ferment, but as it starts ti slow down, it produces less heat.
2) Leak in bucket lid. The seals between the buckets and lids can have leaks, and are the preferential path for CO2 to escape. When fermentation is at it's height, the airlock may show activity, but as it slows down, there may not be enough CO2 to satisfy the leak and the airlock.
3) It's done. Sometimes it happens quickly. I've had a Belgian finish in 2 days before now. Would still recommend leaving it for at least 2 weeks.