What is happening is a worn Trans tends to sag in the area where the input mates to the mainshaft, when you push the clutch the input "should" stop turning as the gearsets are constant mesh excluding reverse it's not an issue for forward gears, reverse is a Dog Gear with straight cut teeth, if the clutch is draggy or the trans sagged in the bearings the cluster gear will still be turning as you try to engage it.
Pulling any forward gear will stall the cluster allowing reverse to engage clash free.
As I said I use 5th as it's a simple pull down into reverse, you don't have to fully engae 5th, just push against the synchros and that's often enough to stall the cluster.