Black did not react to , leaving White with a or . "Completing the joseki" at is overconcentrating White's forces, since it doesn't react to a destabilizing Black move at a.

Slightly better is "completing the joseki" when the whole board position requires another move, locally or globally. Still, it's putting appearance over purpose.

Why do players do it?

They have learned the joseki or seen it in practice a number of times. They remember the shape but don't remember the purpose. They get themselves into a comfort zone by playing a move they (think they) know, when they don't really know what to play. No doubt I still do so today myself.

Root cause: substituting thinking with recognizing shapes.

Playing premature endgame

In particular: early monkey jumps.

Why is it wrong?

Playing endgame moves when the endgame hasn't started, will often result in strengthening the opponent and/or weakening your own stones, by reducing your liberties or improving their connection and eyeshape. This removes your chances to harass groups in the middle game and creates chances for the opponent to harass your groups.

Why do players do it?

Endgame sequences are set patterns, in a similar way as joseki are. Moves like a monkey jump "jump" to the eye and it is hard to resist playing them when you see them. After all, what can be wrong with gaining points in sente? Well, in the endgame that's the purpose, but in the middle game it's more about health of groups.

Root cause: valuing the size of potential territory over the health of groups.

Invading after reducing

Also known as "eat your cake and have it too".

Why is it wrong?

It's pretty clear: if you want to invade into the opponent's sphere of influence, the more room you have, the easier it will be to live or get another advantage. If you first reduce the area, the room to live decreases. Also, through the exchange of reducing, the surrounding position will have become solidified and you can no longer threaten it from within.

Why do players do it?

This one is not so easy to explain. I think players first go through the motions of delimiting their territories and find out too late that the relative sizes are to their disadvantage. Then they invade in a desparate attempt. Ironically they are sometimes rewarded for this bad approach, if their opponent is not too good at life and death themselves.

Root cause: a lack of positional judgment.