Search block in sidebar pushes subsequent block to the side

When the Search block is placed in the sidebar, above any regular block, the regular block is pushed to the right. Please see the attached image. Looking at the CSS, it's not immediately clear as to why this is happening. Both div#block-search-form and div#block-block-1 (for example) are set to display: block, and the former is not floated, although one of the elements inside of it is.