When one avalanche triggers another avalanche some distance away. The second avalanche releases due to the disturbance of the first.
When an avalanche descends, especially in very unstable conditions, it can trigger avalanches on other nearby slopes. Don’t confuse sympathetic trigger with related terms of “remote” trigger, meaning an avalanche triggered by a person some distance away or “stepping down” where a smaller avalanche breaks down into a deeper weak layer. Sympathetic triggers usually mean that the snowpack is especially unstable and you can trigger other avalanches easily.