this is all speculation and theory on my part so keep that in mind...
I think that any tank with a high population of a single form of life could experience a similar "sudden crash", much like what rev went through. If something happens in the tank's chemistry that causes that particular life form to die, the whole coloney will die and the resulting toxins/ammonia/unknowns released as a result, in a large quantity can have any number of negative affects on your tank.
In rev's case, he had a high pupulation of xenia. to his best guess, something happened to cause the xenia to die. Since all of it died at the same time, the abundance of "whatever" resulting from their death caused a chain reaction. their chemical release (again, toxins/ammonia/unknowns) triggered other life forms to die with caused additional and perhaps different chemical releases, etc. and things just get worse and worse.
so to say xenia caused rev's crash could be at least partially true, but a more correct answer is likely that something caused rev's xenia to crash, which started a chain reaction. JMO
But to say xenia in general can cause a crash, is no more true (or false) than saying zooanthids can cause a crash. Given similar circumstances, zooanthds could quite likely cause a full tank crash. If a tank has a large amount of zoo's and something detrimental to zoo survival occurs in the tanks chemistry... they all die, at the same time. The same negative domino effect could then occur as well.
but, as I said, that's just my theory based on the observations given.