It was an angry sentiment - a more experienced cyclist yelling at an inexperienced rider who had just committed one of the "cardinal sins" of pack riding: by changing lines dramatically, he had cut someone off, causing a crash in the back of the group.
What caused the accident was fairly simple - a guy pushed himself well beyond his capabilities, and in exhaustion, he began making poor choices. In this case, the newbie had ridden as hard as he could up the left side of the group, from the back, passing everyone in the group, in order to get a better placement in the peloton (having always heard that the best and safest place in the pack is near the front). He had gotten about as far to the front as he could (or dared to go), and then dove sideways into the group to seek shelter from the wind.
Unfortunately, I see this happen all the time: a guy rides up the left side of the road, passing everyone, but in doing so, he burns himself out. By the time he actually gets to the front, he cannot hold his place there. He burns all of his energy flying up the yellow line (or over it), jumps onto the front, and blows up. Then everyone around is forced to work around him as he is shuffled to the rear again (if not off the back entirely).
The result of this newbie's move years ago - diving into the group, bumping someone's front wheel as you can imagine - was like a bomb going off; cyclists swerving and veering off the road like bits of shrapnel in every direction.
This rider had inadvertently overlapped his rear wheel with the front wheel of another rider. But here's the thing: that cyclist (the one whose front wheel was hit) did not go down! He was a strong athlete with good bike handling skills and many years of experience. He flexed his line and adjusted his speed to take the blow.
The trouble arose farther back.
Reactions to changes in speed are exponential: when the First person slows a fraction, the Second must slow twice as much. The Third must slow twice as much as the Second (thus four times as much as the First), and so on and so forth. The person on the back is forced to slam on the brakes, then sprint to catch back up to speed.
We call this the accordion effect (sometimes the slinky effect - same idea, but "accordion" also references sound... and in truth, this action is most like particles of air in sound waves or compression waves). It's most obvious when slowing for a turn, but it happens on hills just as often, and it can even happen on a flat, uninterrupted road during which someone hesitates to drink from a water bottle!
So by abruptly ducking into the group, the newbie had caused a chain reaction that ended in disaster several riders back.... but the cause was undeniable.
Fortunately, no one was severely injured (bruises, scrapes, scratched components), but the new guy got a good tongue lashing from the stronger cyclists up front. "Know your place," one had said, and though it was sharp and unforgiving - he brought up a good point.
All too often, people breeze through advice columns and forget to read why. Why do cyclists ride in the front of a group? Who is up there and why are they up there? What makes a peloton work and what makes it collapse?
Just as every rider should choose (or create) a group ride based on abilities (being completely honest with oneself), every rider should also choose a place in the pack based on personal experience and comfort.
If you are just hoping to hang on - go to the middle: there will be fewer and smaller surges, fewer gaps, and plenty of draft. If you are a strong, steady rider who can set pace - ride on or near the front. If you are planning to lead the ride - please be near the front at almost all times (it's hard for the caboose to steer the train).
The point is: "know thyself." Be honest with your capabilities and comforts. Yes, you cannot grow without pushing yourself sometimes, but a group ride is not the best place to go well beyond your limits. Consider everyone's safety first. In truth, you will gain more by sitting in and working on pack-skills than by killing yourself, blowing up, and riding home alone.