What can have killed these frogs?

[Primrose]

I have a small "sink pond" at the edge of my vegetable patch as a temporary refuge for frogs. A few days ago when I checked it, it had no wildlife in it and the water is very clean. Today when I checked it again I was sad to find 3 adult frogs and one tiny frog (probably last year's baby) floating dead of the surface. They have obviously come out of hibernation in the past 3 or 4 days and then died. My other small pond where frogs normally made has no sign of frogs yet. Can anybody suggest why these frogs have died so unepectedly? None of them showed any sign of injury. I'm worried I may now not get any frogspawn this year as our garden is securely fenced and I have no idea how many frogs over winter in it.

[glallotments]

Has the sink pond got easy access out of the water? If frogs can't get out to breathe they can actually drown.
If a pond has steep sides it's possible that they can jump in and then fail to find anything to land on to climb back out. Also if the pond is shallow and they can't get out and find somewhere to hide I suppose it is possible that they could freeze to death.

[oldherbaceous]

I was wondering if they were exhausted from hibernation if they might have drowned.

[Primrose]

There are some bricks at one end of the container where they can climb out and the water hasn't frozen over the past few nights so it's a bit of a mystery.