Cirno doesn't understand what winter hiding means while other yukkuri only ridicule her. So Cirno thought of her own answer.
A scum couple take over other yukkuri's house and food and treat the real owner as slave. They're almost dead when a Leti attacked, but their barrier is destroyed and all food is taken by Leti.
Cirno found them. Marisa planned on taking over Cirno's home too.
But both side misunderstood each others. Cirno thought all yukkuri is like herself, strong against cold and can eat the abundant snow. Marisa and Reimu thought Cirno is the same as common yukkuri, hiding in nest and eat gathered food.
Cirno think winter hiding means hiding in a hole in the snow. That's why she never saw any other yukkuri in winter. Cirno is also perplexed to see Alice now hiding in snow hole.
After scum couples jump into the hole, Cirno go to find Leti again while the couple angrily look for way to break out.
Then decided to dig up to the surface.
That's when Leti passed on them and almost crushed them both. Now Reimu and Marisa is fated with two choices. Die freezing, or die of in troubles.