And answer is: problem was width of one element – #joinform.
I have decreased width to 463px, and now all works fine.
It seems that overflow: hidden didn’t work: iPad tend to ‘show’ hidden content, and to ‘push’ elements on page, if needed.
So, sum of widths shouldn’t be more than 100% in any case, iPad doesn’t tolerate it.
Also, i got good advice related to iPad testing – Google Chrome emulator (included in latest version) is very reliable (errors was visible in emulation).
I hope this will help to someone.
solved Strange iPad portrait and landscape view issue