I agree with the summation that during the hot weather it is common for more urine to be excreted due to the birds drinking more fluids. Now, put on top of that the extra fluids in the feeds they are eating and they have more fluid consumption than normal. It also depends on how soupy your feed mix is also, as they have to eat more fluid just to get to the feed, so you can try a dryer mix if you aren't already and it can help a little.
My birds went through some messy butts during the hottest weather this summer...about half the flock, but some did not. Now that is over and will probably come around again during the hot weather next year also. Every year about this time I have a few hens that have more liquid in their stool resulting in messy butt feathers but it normally passes before too long.