I finally made some time to do this post.  I think I may know what the cause is of some cases of "Sudden froglet death syndrome."  I noticed some froglets will keep trying almost nonstop to get out of their containers. It appears that they may exhaust themselves in the process and possibly drown.  My mitigation strategy has been to place them in a container with a thin coating of water on the bottom, so that they don't become dehydrated, but not enough water for them to down in.  Seems to have helped prevent some cases.

I enact this measure when I see a froglet appearing to struggle such as not getting on the rock and floating upside down at times.