I figured out my dogs trigger and it's STRANGE

OK so I had my suspicions what her trigger is but now I know for a fact.... It's the other dog trying to jump on the bed/ couch and falling down.

Three times now my smaller dog has done this, three times now the bigger dog has attacked her for it.

We recently found out the smaller one is blind in the one eye so we got stairs but she hasn't quite figured them out yet and she still prefers to jump but every now and again, her aim is off and she falls. And then the bigger dog just leaps in to attack her.

This is such a weird trigger and I have no clue how to stop it!