Not sure...could be some sort of poison from spray's or just old bees dying?

Keep an eye on this swarm, if the queen is laying, or start to lay
in about 2 or 3 weeks if you have a new one coming,... you should be OK.
Perhaps post this on Beesource.

You may want to feed these bees with commercial food patties. As a quick feed mix a little water in sugat to make a sugar patty, just barely damp. Put this in a saucer or jar lid an slide it into the front entry. Ideally you can get a feed spacer , or use an empty super (no frames) on top of your hive then place the sugar saucer on the top bars. The spacer or empty super will give the bees room to get to tthe food. Also you can dissolve one cup of sugar in one cup of water put that in a clean spray bottle and spray your bees with this food even to the extent of lifting out comb frames and spraying the bees and the wax. Works great to give the bees a food boost.

15 dead bees I wouldn't worry about. Swarming is probably pretty hard on some of the bees. But it could be a pesticide but there isn't much you can do about that.

Jeff