Two things spring to mind...my mom tells all the time about a Lab at a logging camp I was in as a toddler, apparently this Lab felt it great fun to knock me down, into the snow, or mud, whichever was there, and to stomp on me as he went by. I know, amazing I even own dogs now, considering...! LOL
But another thing comes to mind, Shepherds sometimes actually knock their charges down to stop them from running. I saw our GSD do it the 1st time, our neighbors were there visiting and the toddler went kind of running towards the road, and Hunter just reached out his paw and whap, hit the child on the back with his paw, and the child went down flat.
When I was younger, I had an Aussie type dog and she would knock my daughter flying if she could. She'd go running by and manage to bump her as she went by and knocked poor Julie down.
So...in my limited experience scope, I'd guess your dog either thinks it's great fun (may interpret your baby's screams as glee) or else is practicing herding instincts, or a bit of both. Sometimes our suppressed herders just do the next best thing they can figure out and it looks really bad or strange to us because the kids aren't sheep, and it's not a grassy field.
But. It sounds like gates, for now, are your friend
And if leashing her to you, keep the leash 6' or less.