When turning cranks backwards, cassette locks up

Edit: Assuming a standard bike, but take clues for internal gears or ebikes.

The rear derailleur keeps tension on the chain when you pedal backwards via a spring on the two jockey wheels. If there is resistance that exceeds the force this spring exerts then the jockey wheels will shift forward, slackening the chain, instead of the system working smoothly.

This can happen from a number of causes:

1. Your jockey wheels have a ton of crud in them and need cleaning. This is the most likely cause, particularly if you don't take good care of your chain. Have a good look at them, and try to spin them by hand if you can get the chain off easily.
2. Your freehub is blocked up somewhere. Try spinning it backwards while holding the chain clear with your other hand. If it doesn't spin smoothly then you need to take it to your LBS to have it cleaned/greased/fixed.
3. Your bearings on the jockey wheels are shot. Sort of the same as #1, but needs a new part.
4. Your chain is rubbing somewhere with a lot of friction. I'm guessing you'd notice this, but I thought I'd include it for completion.
5. Something else is causing more friction than the derailleur spring can compensate for. You should be able to find it by looking at where the chain is stuck?

Hope that helps some!