That's the most likely reason.
Cats without indoor facilities that come with a Human to keep them clean will tend to poop on the edges of their range to a) mark it out and b) announce their presence in the area, especially if it's not covered to damp down the "strength" of the message. More so if the cat is an entire male; at this time of year they're on the look out for girls and pretty keen to make sure everyone knows they're around.
If the cat isn't neutered, maybe have a word with the neighbour about sending his boy off for a pompomectomy as it will definitely ameliorate the situation. Output from the rear will also be far less odoriferous, a neuter boy is far less likely to wander, and is at a much lower risk of contracting FIV and FeLV as these are spread by both sexual contact and bites / fighting. Oh, and it totally eliminates the risk of testicular cancer of course.