not all things that happen to us is due to our Kamma, sometimes it is disease (four of the reasons), Kamma, carelesness, changes in season, and harsh treatment as found in SN36.21
now although this list does include things which could be the result of past Kamma, not all are necessarily the result all the time.
I take season to also include natural cycles that result in storms and earthquakes & harsh treatment being what we or another does fwit.
Yes, not everything that happens to us is due to our Kamma. But if it wasn't for our kamma, nothing bad would happen to us.
Depending on the perspective, one could argue that everything you suffer is due to your kamma. The kamma that brought you into this life with the many dangers and suffering - the karmic process (kamma-bhava) is just explaining that it's kamma why jati / birth occurs. If we wouldn't have the particular kamma to be born (in this or a lower realm), none of this would happen to us. From a level of ultimate truth, every kind of suffering can be prevented by reaching parinibbana and never being born again. No birth, no suffering, right? That's the deathless.
The Sivaka Sutta probably needs to be seen in the context. Moliyasivaka the wanderer heard from the brahmans that "there are some brahmans & contemplatives who are of this doctrine, this view: Whatever an individual feels — pleasure, pain, neither-pleasure-nor-pain — is entirely caused by what was done before."
Of course this means that Moliyasivaka asked the Buddha about whether pleasure, pain, neither-pleasure-nor-pain are determined in this life for a person by the past.
Which is not the case. And while once birth (by our kamma!) occurs, suffering is just inevitable to happen, not every pleasure and every suffering is caused by what was done before on a more basic level. Like weather, diseases etc.
So, on a meta-level kamma is the reason why we are born and therefore is also the reason for the suffering in this life. I think that this might be a standpoint from which people can argue that khamma is the reason for all our experience. But this easily gives the wrong impression if not set in the right context. People don't die in a tornado because they did something bad in the past, but rather because they're born in this realm where tornados are part of the natural dangers. It can be easily misunderstod if one would say "this person died in the tornado because of her/his kamma" - It would be more accurate to say "this person died in a tornado because all people who are born, die sooner or later."
That's how i understand it. Please correct me if i'm wrong here.