points in BM affect pet agro?
Lots of things effect weather you or the pet gets aggro.
But straight to your question the more points in BM the stronger your pet and that can effect aggro. Also other traits like 30% bigger crits could have an effect. DPS is not the sole sorce of aggro in this game, but it is one of the sources. Others things such as first hit, closest physically etc all play a part as well as the mob type.
I have 30 points in Beastmastery. My pets still don’t always tank. I think a key is Risen have never allowed the pet to tank against them, and at level 80, that’s all I fight anymore.
That said, even with 30 in BM, against anything else besides Risen mobs, a key to having the pet tank is to make sure the pet attacks first. Also make sure the pet is doing damage.