I have experienced leaning to one side on our tandem which occurs due to a weakness in the lower back. For the most part it happens on a longer ride such as over 60 miles. It can be prevented by stretching occasionally. I also have this problem of leaning when on my road bike on double century rides when I have not stretched at check points. Here it can occur at 170 miles, sometimes sooner. Finally, I have been working out for one hour three days per week emphasizing strengthening of the core muscles. Between this and the stretching on rides, the lean has been for the most part eliminated.