can somebody tell me what is the cause?

For 2 of my countries when I first play turns each day a large part of my population is lost!!!

BUT... I can't see the reason? I did not lose land, lose buildings, lose tech, nothing saying spies poisoned the food or stole tech, etc.

What am I missing/not seeing? :/

You put res tech on the market (as soon as it hits the market, it no longer counts as your tech). The turn after you place it, you lose pop for a bit.

K4F I was thinking that, but I have not sold enough tech to lose so much pop. :(

Any amount of tech sold will cause you to lose pop. How much % of pop are you losing? Even my techers are losing 30k+ a turn, a casher or theo would lose much more if they are reselling.

only selling 50k res tech, next day -70K pop first turn, -55k, -40k, -30k next few turns :-(

The one thing I can guarantee you, is it isnt random. Judging from your numbers, that makes sense. It sounds like you have 200k res tech total, so you are lowering a massive portion of an already low percentage. Selling 1/4 when at max barely matters, you lose 4%. Selling 1/4 when you are 1/2 of max lowers you by like 10-15%.

Yeah, the more land I get the bigger those drops get....scary big

And here I thought I was losing my pop because I am old...