Sometimes cyclists go down ...
When this happens, particularly in a pace line, everyone should review how it happened and what, if anything, could have been done differently to avoid the crash.
Sometimes it is just simple equipment failure. And sometimes that equipment wasn't maintained sufficiently.
Sometimes the cyclist going down is at fault - nothing anyone else could have done to prevent the mishap.
And sometimes the group, or at least a few others in the group could have made a signal or voiced a warning that may have prevented the crash.
It's well worth the time to think through a crash event and consider all possibilities of how to prevent another one. We're all out to enjoy our time on the road and helping to keep each other safe is a great way to ensure we have fun doing it.