Bulk priming test turned fly trap

I am currently testing the Krausen Wort method out for bulk priming and the first test I had going I put on the kitchen bench so I would remember to give it a good shake every now and then.

This morning I found that a few flies had gotten in there, four to be precise. I didn't close the lid on my shaker so I guess that is to be expected.
However what was a bit strange was that they were all dead. The last time I checked on it was last night and it was fly free.

I am now pondering what might have killed them that quick, co2? sugar overload? the yeasties?

In my meager experience, flies usually do not die that fast when they pop into my glass of softdrink leftovers.

Any thoughts?

I like the thought of carnivorous ninja yeast, but they probably just drowned.