You should be able to right click on the bin field and select clear sort. Hard to tell what else is going on without the marks/filter cards.
Like Adam mentioned "right click on the bin field and select clear sort". If that doesn't works there is one thing that I do is use a calculated field with Rank function to rank your field based on values. Then place that on the rows or column shelf just before the actual axis field and uncheck show header. That works very well for me but adds a table calculation which might slow down the performance of the dashboard in case you are using a huge dataset.