Couple of things to consider:
Is the water in the nipple waterer fresher and from the house supply coming from municiple source? It may have more chlorine than water that has been outside in the open and exposed to light which allows the chlorine to evaporate out.
Could be a temperature difference: Is the nipple source water exposed in the sun at any point? It may get hotter than the deep water in a bucket. Or if it is cold where you are, the nipple water may be colder and the bucket water warmer.
If the system is new, the chemicals from glue or plastic might still be getting into the water, especially as chickens dont drink so much as to keep the water from standing in the plastic for a while. New plastics offgas significantly and do so more when heated. If you have pvc piping or garden hose, it could be adding a very strong taste. Even new water pitchers of plastic have a discernable plastic odor, and humans have the weakest sense of smell in most of the animal kingdom...imagine how that must smell to an animal with a far superior sense of smell! Probably like a trip to the dump does to us
If you have an auto feed waterer, try flushing it out and catch the first couple of cups of water...smell it, check the temp, or even taste it like you're doing a wine tasting and make sure nothing funky is going on.
Additionally, if you let them out in the morning and put them in at dusk, it could be that they just dont eat or drink much at night, and so are thirsty when you let them out.
I only have three 6 week olds so I use the one quart nipple waterer. They dont drink much at night and I move the container out of the coop when I open it up in the am. They do spend a lot of time drinking in the first half hour out. Together they only seem to drink about a cup and a half a day, so you actually might have water standingfor a while in your system if it is an auto fed waterer.
Of course, it could just be the draw of novelty or that competetive chicken nature...