Having the cable too tight and/or the hub adjustment too tight does exactly the same thing as starting to pull in the clutch lever. With the clutch already starting the release process BEFORE you ever pull on the lever, it will be prone to slip. 5th gear used to be square cut and it would whine. There were enough complaints that the gear is now cut at an angle like the other gears except for 1st. This puts a side load on the shaft and if there is insufficient free play in the cable and hub adjustments, that side load displacement will start to release the clutch. I don't know if there is something about 5th gear that puts extra side load on the shaft but slipping in 5th gear is is one of the first signs of the clutch badly needing an adjustment. Someone not paying attention can quickly destroy a clutch when this happens.