Not of the ninja variety. Note that this is an origin, not the origin.
The nature of turtling
When playing, most people want their character to do well, not be eaten by the grue, not be tortured by random demon lords, not lose all of their loved ones, not remain a dirt-poor loser and so on. The effect happens due to character identification or even immersion.
Turtling is the same thing, but taken to an unhealthy extreme. Namely, avoiding all sorts of risks to have the character be safe, where the category “risks” includes plot hooks and other story-making opportunities, as well as anything resembling a mystery.
What follows is a description of a bad system. It can be worked around, but on system-level, the desired outputs fight each other, which is a bad thing. Playing in roleplaying is usually made enjoyable by at least two things: Interesting game, which usually requires that there are conflicts in the game, and identification with a character. The inherent conflict between these two is obvious. There are several ways to handle it.
One way is for players to always avoid all trouble, creating characters optimised to survive whatever happens to them and even triumph over such challenges. The character is practically untouchable. Not rarely have random green-skinned or undead creatures killed everyone the character ever loved and now the character’s life is only fueled by the desire to revenge them (and get rich in the process). The GM’s role is to challenge the character and engage it in something resembling a story. I call this a bad system. This is because the player wants to create a character who is actively hard to engage (in the long run), which makes the GM’s job harder, which is likely to reduce the quality of whatever story does manage to manifest, which usually reduces the player’s enjoyment.
Second way is that the player creates a character that is vulnerable in some way that helps the GM in engaging the character. The point is that any decent GM can see this flag and target the character there. This makes for less contrived stories as the need to use artificial plot hooks is reduced. Orcs killed your family, save for those of your age, whom they kidnapped, and you are out to discover and rescue them. This gives a plenty of material to the GM: Rescue missions, your sister who internalised to orcish culture, grabbed control of a tribe, and is now attacking the human lands, general orc slaughtering. All good fun. The downside, for the player, is that the GM might misuse the vulnerabilities of the character. Arbitrary killing those the character is about to save might provoke some interesting roleplay from certain players, but it is more likely to simply annoy. This I like very much.
Third way is to give the player a reward of some sort for hurting the character (often in specific way). Fate points (by whatever name) are the most common method, though others exist. This means that the player can both create interesting story material and play to the best outcome for the character, or at least not play to the worst. This often works.
The outlier methods are removing character identification or character ownership. Another is ignoring the story dimension entirely.