Despite the terminology, "ingrown" doesn't mean the nail grows in the wrong direction. The lunula continuously emits keratinocytes in a single direction; these form both the nail bed and the nail itself.
The "ingrown" phenomenon occurs well after the nail has formed (it's getting pushed out from the lunula end) and is due to a combination of your toe's (hallux I assume) ideosyncratic geometry and environmental conditions, likely, as another commenter pointed out, how you innocently cut your nail.
Sorry for the pedantry but when I worked in drug development I used to research the nail unit, which, it turns out, few people do.
The most interesting thing I've learned about nails is that they're now thought to be part of an organ — the enthesis organ [1], which is the tissue structures around the site where the tendon attaches to the bone. This is relevant to spondylarthropathies, some of which show up as nail changes many years before enthesitis occurs.
The "ingrown" phenomenon occurs well after the nail has formed (it's getting pushed out from the lunula end) and is due to a combination of your toe's (hallux I assume) ideosyncratic geometry and environmental conditions, likely, as another commenter pointed out, how you innocently cut your nail.
Sorry for the pedantry but when I worked in drug development I used to research the nail unit, which, it turns out, few people do.