Hello! This is a feature of the theme you're using.
"Responsive" designs are ones that adjust the look of a site based on the size of the window used to view it. The idea is that people who use smaller gadgets (such as phones or tablets) to view your site have limited space to do so, so the display is adjusted to highlight the more important parts of your site.
For example, in your blog, when the display gets too narrow, the "post/page" area stretches to fill up the whole screen so your visitor can read your post more comfortably instead of having to
after a few
That's why the sidebar widgets are pushed to the bottom. To give space for the content of your post.
This is actually a good practice for a site's design. Is there a particular reason why you don't like this behavior for your blog?