My 10 gallon has been up for 3 years, with RBTAs in it for 1.5 years. In my case, there were really a couple things that caused the crash. The biggest was a move last summer, followed by less than meticulous maintenance. Couple that with adding a new CUC to replenish for attrition, and that's when things started to go bad. I can't pinpoint any one thing other than my lack of maintenance as a primary cause, though...
And I'm offering my experience for others to learn from. The plan all along has been to eventually upgrade to a larger tank. Even after my Nuvo 25 Lagoon build, I'll be planning for a much bigger tank in my basement, so in my mind at least I'm not justifying keeping nems in too small a tank for too long. But that's not really a fool proof plan.