Friends of mine purchased two nuc's some months back. The two where splits from a good little hive, very quiet. One had the queen while the second had eggs and the produced a queen successfully. Both splits did well - one naturally a little slower.
The owner spoke to me today. One week ago both hives where inspected and where doing well. There was some honey, eggs - good working hives. A few SHB where observed.
Last Saturday both where normal hives as far as could be obseved. Come Sunday one hive was dead. On closer inspection the owner reports that there where very few bees, no honey, a few bees still emerging. No unpleasant smell with frames and foundation clean.

There where a few hundred dead and dying bees in front of one hive. They seemed to clean each other.
The second hive so far is healthy and strong. No unusual activity ( like robbing) was observed.

I have some ideas what could have gone wrong but would love to hear the opinion from others. It is autumn here but still warm. We had a lot of wet weather this year but the last week was mostly fine.