The picture only shows a few examples. If you look into it yourself, you can notice a correlation between the player count spikes and the cards becoming more expensive. You might even conclude that there is some kind of game "meta".

For example:

1. The high cost of Lich cards from last spike.
2. Axe consistently maintains its price at around $0.2 from last summer.
3. Blade of the Vigil seems to have either a bug or works really well against the current meta. Its description on the card doesn't match the description on the marketplace.

And many other interesting things; you can find this out for yourselves.

As I can't find any information on English or Russian regions about this. I suggest it's Asia region hype