I'll start.
The number one cause of antimicrobial resistance is antimicrobial use. Antimicrobials are the “A” in AMR. One could argue that the only cause of antimicrobial resistance is antimicrobial use.
Without trying to be too pedagogical about it, I will just say that as I was teaching students yesterday, I said that if we went just outside and went to the lawn, there was a war being waged between bacteria and fungi. The fungi are defending themselves against the bacteria with antibiotics that they produce. The bacteria are defending themselves from the fungi by developing resistance mechanisms. That war rages on in many places.
We, as humans, don't tend to have many fungi in or on us, but we have many bacteria. When we do get exposed to fungi or the antibiotics that they produce, we use them to kill bacteria that cause us problems. Some of the resistance occurs out in the environment and comes to us, but there really is no major cause of antimicrobial resistance other than bacteria being exposed to antibiotics.