Basically the categories button will repeat the buttons animation if the mouse hovers them after dragging the buttons to somewhere else, this is more noticeable in themes like adapta or materia. Steps to reproduce: 1.-Open Whisker Menu 2.-Click and drag the buttons to somewhere else, i.e the desktop 3.-Stop holding the mouse button 4.-Hover the button that you dragged 5.-The animation of the button will reproduce as if you clicked it | It doesn't matter if you close whisker menu and reopen it, it will still play the animation when the button is hovered. | You can actually do it with multiple buttons making the whisker menu lag for a sec.
Graeme Gott referenced this bugreport in commit 00d39d44fda2252250af774e11a68135425c9224 Fix category button staying active when dragged. (bug #14419) https://git.xfce.org/panel-plugins/xfce4-whiskermenu-plugin/commit?id=00d39d44fda2252250af774e11a68135425c9224
Fixed in 00d39d4.