My stand 'collapsed' one day... JX-8P on bottom, DX7 IID on middle, JX-3p On top.. all within the quoted weight tolerance per tier, Ok I was kinda leaning on it too at the time!
The Problem? The 'quick release' lever in the middle front lower (and two 'toothed locking discs') that hold the whole stand (double X braced) at it's height was actually not tight.. came this way from the factory. It's loose to allow you to snap open the plastic lever, adjust, then close again (fairly tight it seems), but the nut in the middle wasn't tight enough and allowed the teeth to lose their grip with a bit too much weight.
So if you have one of those types of stands, get your socket set out and tighten it up once it's 'adjusted' to the right height (now i'd have to loosen the nut to snap the lever open for adjustment which is fine by me, and a lot safer). If done securely the lever will be impossible to 'snap open' (or even move at all).
Luckily no damage done to any of the synths except for a tiny scratch on the DX7 by the 'power on/off' graphic.