Inside the humidifier, there should be a pan that the drain hose connects to. There's only a few possibilities I can think of, for water to reach the location you've highlighted.
Leaking connection between drain and filter frame
Check the connection between the drain fitting and the filter frame. Make sure the drain fitting is properly attached and aligned in the filter frame, and that water is not leaking from the bottom of the filter frame where the drain fitting attaches.
Overflowing filter frame
Make sure the drain pipe is clear, and that it can handle a volume of water without backing up. Make sure water can flow out of the filter frame without restriction. Check for mineral deposits blocking the drain.
Make sure the inlet is not overspraying the tray. If the water is spraying too hard, some of it could find its way out of the filter frame.
Make sure water can flow through the upper tray without restriction. If the upper tray is blocked up, it's possible water could find its way out of the filter frame.
If the filter is slightly misaligned, it could potentially give water a path out of the filter frame.