As a Wix Designer, I sometimes run into little Wix behaviors that I can't figure out. In the above image, my Wix dropdown menu is dropping "up" instead of down, like dropdown menus usually do. I couldn't for the life of me figure out why, until I got a little help from my friends in the Wix Design Experts group on Facebook.
At first I thought it was a bug or some odd setting I set unknowingly – but as Avi pointed out – it's not a bug – Wix menus will automatically detect if the menu is above the 50% height of the page, and, if lower than 50%, the submenu will drop up. If the menu is above the 50% height it will drop down like a normal dropdown menu.
So there you have it!
In the above screenshot you'll see my menu is lower on the page, below 50% of the area on the page, so it drops "up." In the below screenshot, I had moved everything up so that the menu is above the 50% mark, and it now drops down as desired. It makes sense that it does this, as you wouldn't want dropdown menu items to disappear off the bottom of the page if a browser window is small.