Shelby's problem was NOT that she was assertive. It's that she was smug.
She openly gloated that she would one day have Riker's job. She was rubbing everyone's face in it. Like an interstellar nanny-nanny-boo-boo.
How so? All she did (At 1st) was privately express her intention to replace Riker, a notion I'm sure she got from Hanson's attitude that Riker ought to be leaving, & she should be his replacement. Note how she didn't assume she had the job, just that she had to convince Picard she's the best to fill the vacancy
That's not gloating. It's a misunderstanding, that if anyone is to blame for, it's Hanson. He's brought her there specifically for that reason, to air her out for a potential new captain. I saw nothing smug about her behavior there, a little overexuberant, & perhaps indelicate, but why would she have to be delicate with the big dope, if she thinks he's done there?
Now, because Riker gets his feathers ruffled about it, & has the dumb bravado to invite her to a poker game, she ended up besting him in, it worsened relations imho. Then he gets his panties in a bunch over her taking a command action of moving up her survey to avoid changing planetary conditions, even though Hanson had already set the precedent that she'd have "Wide Latitude" to conduct affairs without the kind of chaperoning Riker sees fit to establish. Again, this is what she was brought there to do, & very likely given to understand she wouldn't have to answer to the entire chain of command on the visiting ship, over every last detail, least of all, the guy who should be leaving. If she must, then she was merely mistaken, again, because of how she was brought in by Hanson
It's after that & some more trouncing by Riker, over how she's managing the job she was sent to do, that she finally gives him the "Your in my way" wake up call. It's not like she was throwing that in people's faces from the get go. It came from somewhere legitimate imho