Why can a site be bogged down in Yandex search results?

Question

Search Engine Optimization

Starting from the second half of September, our site began to throw up and down in Yandex results for the main key phrases (we monitor 31 search phrases). At the same time, everything in Google remained as it was.
Prior to this, the situation in Yandex was also more or less stable, the average position for phrases was 10, for the most relevant ones - 3 ±. Now every day it floats between 15 and 20 positions, and the main phrases have moved down to the top 10 level.
I didn’t do anything special with the promotion, except for gouging through the bulletin boards in the machine.
Schedule of changes in issuance:

Answer 1

Drunk CEO of Yandex fell asleep on the keyboard.
Maybe your site is seasonal? Until the end of September, everyone needed flip flops. Slippers are not worn in winter.
I generally stopped believing in all sorts of Yandex ... there now everything is for money. Do you want to top? Pay.
As they said:
"Make websites for people!"
There was a site, not for people, picture, text = attendance 100-150 people a day
Made for people - add comments, rating, convenience - the site died. 10-20 people per day.

Answer 2

The site is not seasonal, on the contrary, by the way, the demand for business plans is growing just in the fall: masterplans.ru