Negligence vs. an accident. One could argue that dropping a haul bag is nearly always negligent. If you’re careful (eg. not negligent) you should never drop a bag absent some material failure which is unlikely.
In most situations, a defendant is required to exercise the same level of care as a reasonable person would in similar circumstances.
Two leashes on a haul bag seems like what a reasonable person would do. Accidents happen all the time in climbing and don't always mean negligence. If someone left a haulbag un-tethered on the edge of a ledge and it fell I'd say that's negligence. But somehow messing up clipping something in happens quite often even when people are exercising care.