As of lately, I have been farming snurlte shells for some fun projects. However, what I've noticed is that the snurtle mound itself is treated as a hostile entity. Given that the snurtles come out of the mound rather close the mounds, what this sometimes means is that once I've killed the snurtle, I've gone and accidently hit the snurtle mound.
Given that snurtle mounds are limited resources, I suggest that their attack priority to be that of either where you have to force attack it (Control + F), or maybe even click to attack it.