Re: 3/8 Alska AMPS = 180
If he has any spinal arthritis, it may be causing him pain when he's in the litter box, so he's going outside it.
How long is he from tail base to chest vs how long is the litter box. If it is about 1.5 times as long as he is, he doesn't have to scrunch up to drop it in the box. Also, is there one side that is lower and easier to step in? That can help too, for cats with arthritis or other pain.
There is also the "smells like the right spot" factor. It can take several applications of enzyme-based cleaners to remove the scent which can result in repeat soiling. Natures Miracle is one brand.