I'm not sure what causes this, but it seems that this bug doesn't need any special precondition. At a certain time when an app icon is added to (or removed from?) the system tray, it grows, occupying the panel. Removing the system tray from the panel and re-adding it causes it to grow even more. Restarting WM/KDE would help occasionally, but it no longer works (since KDE 4.8, I guess).
"~/.kde/share/config/plasma-desktop-appletsrc" reports "geometry=143,3,2505,32" (for example) for "systemtray" applet and it keeps growing.
Now I have the whole panel occupied by the system tray (its icons are no longer visible as well) :-(

Reproducible: Always

Steps to Reproduce:
1. Fiddle with apps that add/remove icons to the system tray.
2. Remove system tray applet from the panel.
3. Add system tray applet to the panel. Repeat steps 2-3 to see the system tray grow.
Actual Results:  
System tray keeps growing, occupying the whole panel and beyond.

Expected Results:  
System tray should grow only as much as needed to contain the (docked) icons.