when would weapons occupy rear hexes instead of front hexes?

[Plane]

Like for example with that horned warrior on the cover of Basic Set characters, the shoulder extension when he's jumping forward his left shield might be in his left hex and his right axe might be in his right hex.

This is generally a bad thing not to have your weapons in front of you to impede an opponent but maybe there'd be benefits to moving around this way in terms of the straight on holding your posture. The mechanical effect is of course important since it determines the hex location of a weapon to attack it.

[Eric Funk]

Another example may be wielding a long spear or pike with a reach of 2 or 3 hexes when in the grip when the wielder has a grip other than one to reach the furthest hex. (so the 9-yard spear occupies one hex behind and one in front?) (B272-3) I do not believe technically these impede anyone's movement. (in Tactical Combat B384+)

[RGTraynor]

Pretty often, for polearms and longspears where you're not set in a battle line against an enemy's line. It's simply a matter of balance: you're just not going to be able to hold a 9" pike such that it starts at your center mass without grounding it.

[Pursuivant]

If you "choke up" on a polearm or spear (i.e., a 1-hex Reach rather than 2 hexes) then that extra length has to go somewhere.

Assume that each yard of Reach not to the front of the wielder occupies (Reach-1) hexes to the user's rear. That is, (Reach) hexes to the front, 1 hex occupied by the weapon & user, and the remainder to the user's rear.

Practically, this might only make a difference when fighting in extremely enclosed areas, like inside a vehicle or a narrow corridor.

As a suggestion, apply a -2 penalty missing yard of space to use a weapon if you don't have enough space to your rear. The weapon's length limits your ability to Parry and strike effectively.

If you're actively using a weapon to actively cover side and rear hexes, that's covered by the Back Strike technique and your ability to make inaccurate attacks to your Side hexes. Back Strike technique could easily be expanded to allow more effective attacks to Side hexes, not just rear hexes. That would model some of the tricks that staff and greatsword fighters can do.

[RyanW]

There are some swordfighting stances that put the point of the sword behind the wielder. Most notably the long tail guard (posta di coda longa in Italian, nebenhut in German, waki-gamae in Japanese).