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.