Why is it doing this?  

RE: Why is it doing this?

you don't give us a lot of information to give a conclusive answer, 
however, it is likely that the right hand end of the model tapers to dimensions that are thinner than the extrusion width, set in the 'Printer Settings>Advanced>external perimeters box (visible in Advanced Mode, and Expert Mode)

We could look for more definitive answers if you saved the project file, in Prusa Slicer, then zip compressed the file, and attached the zipped file to your next post. 

However, ways to address the likely cause
1, re model the project to be slightly thicker at the right hand end. or
2, re slice the model for a thinner nozzle, or
3, turn the model 90degrees so that the Right hand side becomes the bottom, use the slice tool to cut a couple of millimetres off the thin bit which will be at the bottom, then turn the model back 90 degrees. (this has the disadvantage of making the model shorter), or
4, set the XY Size Compensation value (In Printer Settings>Advanced (Visible in Expert Mode)) to a larger value, say o.2mm
this will artificially enlarge the X and Y dimensions and probably restore your straight edge, but this the disadvantage of making the model slightly larger, and may  affect the 'fit' if this model is used in conjunction with other components
there may be other options such as rotating the item so that the thin edge is horizontal which would probably  increase the model's resistance to damage on the thin edge, and allow the thin edge to resolve, bur would probably  cause issues with support... 

anyway, you have a couple of things to try. if you need more assistance, please attach the .3mf file in zipped format

regards Joan