Strange (?) Jumping Experience

I've jumped cars a handful of times. Classic red dead, red donor, black donor, black ground. Start donor, start dead.

Recently my old car (which has been sitting) didn't like that very much. A couple times it has gone completely dead - no lights on dash or anything. That sequence hasn't done a thing, even letting the donor run connected for 15 minutes. I am confident I had good connections - saw small sparking on bare metal ground each time. But maybe you'll tell me I'm wrong.

What finally worked was removing ground and replacing it. Instantly saw lights come back on and started no problem.

Does this sound "normal"? I've got the car being looked at to fix whatever underlying electrical issue is going on to cause the battery to drain, but I'm curious if anyone can explain why removing and replacing ground did the trick.

Thanks!!