A square knot undoes...

See the square knot....right over left, left over right (or left over right, right over left...same thing).

Take two adjacent tails and separate them.

Keep pulling.....WHOOPS, you now have two half hitches sliding along a single thread.

Square knots are not good under pressure, if the tail separates from its mate!

Granny bows have this problem, too, but they are even more likely to get into this state.

If you tie a third knot on top of a square knot, or a bow, it will have less of a tendency to come apart. For baby wearing, I tied a square bow (as opposed to a granny bow), and then tied the bow loops into a double bow.

Don't trust a square knot or a granny knot whenever you are wearing something that matters (like a mai tie or wrap). Use a bow on top, or fasten the tails to their mates in some seamanlike manner that will hold (in order for the square knot to fail, the adjacent tails must separate, so there are many hitches that will hold them together, that are easy to take out on purpose, but will not fall apart easily).