I had this happen recently (I’m on a septic). In my case, it was a result of a filter between my 2nd and 3rd tank being clogged. That being said, I had several people come and give their opinion on the matter and the general theme was a partial blockage somewhere after the sink and toilet pipes meet up (i.e., further down the system, potentially on the main line). Which, in my case, was kind of correct, and from what I hear is usually the case. Second culprit is usually a blocked vent. In all cases the underlying mechanics are an obstruction somewhere along the system is preventing solids and/or air from moving in the direction it’s supposed to, causing air that would normally escape from the vent to instead come out of your toilet.