Blades disengage

I just bought a lightly used LT 1050. I encountered a problem during my first use of it that I can't find covered in the manual. The blades will just (often) disengage at seemingly random times during use. I would note that they do not disengage when in reverse, as the manual says they should, but at random times while mowing, for no reason that I can see yet. I assume something is out of adjustment, but there is nothing about such adjustments in the book. Ideally, I would just like to disable whatever the connection is between movement and blade engagement, but in any event, I need to fix this. Any ideas?