Boxers are combatant dogs and this may well be the beginnings of dog aggression. She may have just decided "I HATE this dog". It happens.
If the aggression escalates I would recommend a crate and rotate routine.
It also could be hormonal. Have one or both of your dogs altered. It is NOT safe to have them both together intact, YES a dachshund could breed to your Boxer...
I could go on and on about how there are so many dogs without homes and how most dogs are not worth breeding yada yada yada but you should already know this.
If the aggression escalates I would recommend a crate and rotate routine.
It also could be hormonal. Have one or both of your dogs altered. It is NOT safe to have them both together intact, YES a dachshund could breed to your Boxer...
I could go on and on about how there are so many dogs without homes and how most dogs are not worth breeding yada yada yada but you should already know this.