Minecraft – What’s the cause of the South/West Rule

This is happening because you need at least one block of redstone to point "towards" the track, you can't just run parallel to it. Hopefully this awesome paint diagram will explain what I mean:

When placing rails, or connecting redstone, connections going South and East will take priority over those going North and West. This is due to those directions always being checked first for connections on each axis. There is nothing that can be done to get around this, other than avoid ambiguous connections. Most redstone schematics will say whether they will only work in a certain orientation or not.

This isn't really a bug as such (it would be impossible to fix, since one direction must always take priority). Here's some pseudocode to illustrate the situation:

checkConnections(trackPiece):
   if blockEast.isTrack:
       connectEast
   else if blockWest.isTrack:
       connectWest

Since connectEast and connectWest are mutually exclusive, and one must be checked first, there will always be one direction on each axis that takes precedence.

The rule has a few important consequences, detailed on the wiki. Most notably:

• Minecarts will favor South or East motion at a "+" intersection because they favor "starting" at the North or West edge.

• Unpowered tracks will curve south or east when they are initially placed at a "T" intersection.

There are other block orientation quirks not directly related to this quirk listed on the wiki too.

Note on South-West vs. South-East

Prior to 1.9pre-4, West took precedence over East. Since that release, East has taken precedence over West.