Chain shifting when back pedaling On my bike I found that when I backpedal, if the chain isnt perfectly straight, it is prone to shift to a more central gear. E.g would be if on the small chainring and the largest cog, backpedaling even a turn or two seems to result in a shift to the 2nd or 3rd cog. I came across a similar question which settled with the answer that it is due to the chainline, which I agree is not perfectly straight, but neither is it so bad that it is considered "cross chaining" and it works quite well in the forward direction.

Beyond just chainline, is it also an indicator of other issues? Chain wear? Cassette wear (I notice that some teeth are angled forward/away from the bike, and such teeth end up causing the shift)?

This is a second hand bike which is 5-10 years old. Running Tiagra 9 speed.
This is pretty much expected behavior.

When you're pedaling forward, the top half of the chain is pulled tight by the tension from your pedaling, and the lower half is held in tension by the derailleur.  The derailleur is also very close to the rear sprockets so it can guide the nicely-tensioned chain onto the proper gear.

When you pedal backwards, you pull on the bottom of the chain which pulls the derailleur forward.  That creates chain slack.  Some of that chain slack shows up on the top half of the chain as the chain links get dumped off the front chainring into space on the way backwards.  If you watch the chain closely when you pedal backwards, you can actually see the top of the chain go slack and flop around a bit.  So when you're pedaling backwards there's nothing on the top side to maintain tension.  A slack chain will flop around and there's nothing on the top side to guide where the chain lands on the rear gears.  So the loose flopping chain is free to go where it wants.

Derailleur-equipped bicycles are not designed to be pedaled backwards. Pedaling backwards is a great way to drop your chain, especially if you do it suddenly and hard.