Hi Rachel and many thanks on your making this theme!
I don't know if I broke something here or if this is confirmable but the button tag CSS seems to act weirdly on mouse over. This is the following code which look like CSS is missing:
<button class="btn" type="submit">
Can you provide more detail?
I added the css you have above and I don't see anything weird regarding the hover behavior.
Ok i found the conflict (even after 3 months...).
When the button was hovered had an ugly gradient.
I had enabled the buddypress-template-pack to make this theme buddypress compatible and it adds some more css.
The conflicting file is .../plugins/bp-template-pack/bp.css
In line 684 (the hover css).
Edit: the best solution might be toggle the CSS off in plugin's custom menu (/wp-admin/themes.php?page=bp-tpack-options)