Two dead nucs

On April 11 I helped a friend find and remove the queens in two of his hives so he could requeen. The queens were from wild swarms and since I'm interested in locally adapted bees, I took the queens and put then in a 10 frame medium with some open and capped frames of brood and some extra bees, took them to my bee yard and put on a top feeder with syrup. Closed down the entrance to about 1/2". It was rainy and in the 40's-50-'s for the following week and today when I checked them, both were dead. The majority of bees in both boxes were on the floor of the bottom board, but some had their heads stuck into cells, as if they had starved. The syrup could have been fermented but I didn't think so when I added it. I had taken it off some hives earlier in the month and put it in a refrigerator for just such a need. What do you think happened? Could they have been robbed out and killed, or if the syrup was fermented, would they have starved and dropped to the bottom of the hive during the cold/cool weather? Although several days were rainy, others, between today and when I hived them, were just overcast but warm enough to forage, and a flow is going on? I am at a loss as to what could have happened and certainly welcome ideas so I won't repeat whatever it is I did wrong. I'm bummed because I have a couple of upcoming trapouts (waiting to hopefully catch a swarm from the two locations before I trap) and had planned to do combines of the trapped bees with the two nucs.