when I roll my bike backwards the gears on the rear change--25, 23, 21 etc....seems like something is not lined upright. It doesn't affect the shifting when I'm riding, nor does it change on it's own while riding. I was just curious why is it doing this and should I be concerned--I have no plan to ride backwards

It happens when you are in a gear in which the chainline is "off". The chain is auto shifting to to a central cog that better lines up with the chainring. If you are in a middle cog, it won't happen.
They all do that.

You mentioned the reason - something is not aligned. The cassette has some width and there's no way all the sprockets can align with either chainring, so, as you point out the chain isn't lined up and will tend to derail towards the center of the cassette.

To understand the problem better, take a look at the chainring's teeth and the cassettes. The chainring has pointed teeth that can pick up a chain even if it's coming from an angle. However cassette sprocket teeth are cut square, and are not good at picking up the chain if it comes from an angle. The cassette is 100% dependent on the chain being guided straight to the sprocket. They do this for faster more precise shifting, and the system works fine as long as the chain is moving forward, ie. from the RD jockey wheel to the sprocket, but it means that the rear will be prone to auto shifting if running backward.

You do not mention what your astrological sign is, with Venus in retrograde any rolling backwards with one's bicycle may not be proprietary and could well be ill advised.
Or something is not aligned properly, one or the other and neither matters if forward motion is unimpeded.

-Bandera

To bring FB's post to it's logical conclusion - Don't roll the bike backwards, or if you inadvertently do just pedal forward a few turns and things will return to normal.

thanks for the explanation

The fact remains that if you turn your pedals backward, or roll the bike backwards and gears change, something is misadjusted. Either the hanger is bent, or the RD is not adjusted right.

No, absolutely not, most definitely not, no.

When turning the drivetrain backward, the chain is going from the cassette to whatever is below -- RD & hanger -- so they cannot affect how the chain is feeding onto the cassette at the top. It's totally obvious if you think about it for even 2 seconds.

OTOH if the OP was reporting a problem with the chain coming off the bottom of the chainrings when back pedaling, then the hanger or RD might be implicated since they determine where the chain would be coming from as it moves to the bottom of the ring.

With a single speed or IGH drivetrain that problem rarely happens .. I walk the bike backwards to get it out the front door entry/ mudroom.

As FB said, "no". When pedalling or rolling backwards, the chain action is at the top of the cassette, where there is no derailleur to guide it, so it tries to "settle" into line with the front chainring. But since the derailleur is still in the original spot, the chain will get resistance as it can't engage the derailleur pulleys properly.

Lean the bike up in 1st or 2nd gear, slowly back pedal by hand, and you can see it clearly.