The reason that this is unexpected you were a security blanket of sorts that could be used for them to assert their own ego self in a way that deprived you of your ability to assert yourself. Now I'm not saying this is true per say just that you learn a habit or stay in a health state that is contributing to the reserved low key stylistion, that everyone expects and suddenly when you assert yourself people are shocked and assume this is aggressive almost when the truth is you are starting to become healthy and set boundaries that used to be trampled on.
This tendency happens from our desire to be people pleasers and be harmonious with people of all walks of like which loses respect from some people who then love to trample on your boundary even more for being unassertive and test you to see what your limits are to establish their hidden rules for you. This is particular true for dominant personalities that love controls.
When you are unyielding, have an opinion and are suddenly able to stand up and assert thy will of course there is going to be tension. They will find this behaviour disagreeable now because it confronts their insecurity and why they are friends in the first place. And when you do become assertive while people may disagree you will change your boundary to what you want and people will respect this without trampling on your boundaries.
The trouble is the backward slope, lick a ricochet, you bounce from assertive to pleasing to assertive to back depending on your ability, health, attitude and comfort level.