@Tearsax they might have a prolonged sickness which they don't know when it will heal, as an illustrator myself I know it firsthand. It can be carpal syndrome or back ache, those two are nightmare for us.
Also some of these mangaka also works on multiple jobs. They might also do illustration gigs outside of their main job.
@Tearsax the delay between the chapters isn't only the chapter being drawn but also the translation by the scan, which makes it take longer than just the drawing process, not only that, but there is more than the drawing happening, the author has to write a script which will be crudely drawn to a sketch by an assistant, that has to be approved by the author, that has to ink it, and I could go on and on, but, the point is it is longer than someone would think at first