If you read that sentence in context, your questions are answered. Here are the previous two sentences for context.
The bigger problem was dealing with big traffic spikes. When a big spike in traffic came in, it created a domino effect that would take take down our entire cluster
Given the article to that point, it is clear what is happening. They are maintaining a steady state of X% (where X is above 50), then a big traffic spike comes along. That traffic load is not distributed equally (my guess is because requests are not created equal) and there is a hot spot. The hot spot fails, then increases the load on everything else. After this repeats a few times, there is insufficient capacity.
In other words at some steady state threshold above 50%, you wind up without sufficient capacity to handle traffic spikes. Nothing in this failure scenario is specific to Rails. It is a well-known failure mode for a cluster under load with a push based architecture (which http requests are).
The fact that you did not understand that description speaks poorly of your problem solving skills. Now they may well have been doing something trivial that is fixable to cause load to be less than it was. But you haven't convinced me that you're the person to be trusted to figure that out.