Q: Why does a zipper open behind the slider?
This is the classic symptom of a slider that can no longer hold the chain closed. The slider's flanges and bridge form a channel sized to the tooth profile; when the body spreads, the spring weakens or the flanges wear, the channel becomes too wide and the teeth separate under load. The zipper closes as the slider passes and then splits behind it.
Other causes include the wrong slider size for the chain, a chain that has worn thin, and a wearer who pulls the slider sideways so that it levers open. On metal chains, a single flattened tooth can also allow the interlock to release locally and then propagate along the chain.
Confirm the diagnosis by replacing the slider with a new one of the same size and hand, then testing under load. If the problem disappears, the slider was worn; if it persists, measure the chain width and inspect for damaged teeth. Do not simply squeeze a metal slider with pliers as a permanent repair: it may work briefly, but the spring and flanges are already compromised and the failure will return.