She may be trying to herd the other dogs - unfortunately this is a common issue with Belgians! I've been discussing this behaviour with another BSD owner on a different forum and the jumping & pushing/barging at other dogs is one of the ways the breed herds.

You may need to channel this herding instinct into dog sports such as agility or flyball. The breed is sometimes described as being "dominant" towards other dogs (i.e. bossy); but "dominance" in the way your partner is thinking, is an outdated concept and you will not find any reputable dog trainers using the term nowadays.

I suggest you look for a APDT trainer in your area and ask for advice

PS: If your partner is a fan of a certain TV trainer, I'd say those methods are highly unsuitable for a Belgian as they are very sensitive and do not tolerate harsh treatment.

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