What does it mean to call critical thinking "self-directed" thinking?

Thinking that is self-directed, self-disciplined, self-monitored and self-correcting — it polices its own quality.

One precisification of critical thinking casts it as a loop you run on yourself rather than something done to you:

• self-directed — you steer the inquiry,
• self-disciplined — you hold yourself to the standards,
• self-monitored — you watch your own reasoning as it happens,
• self-correcting — you fix your errors when you catch them.

The catch is that this requires both affirming strict quality criteria (agreeing they apply to you) and mastering them (being able to actually apply them). Wanting to think well is not enough; you need the skill to notice and correct your own slips.