The belief in the trade is that this is related to heraldic crests.

On these, a stripe going from top left to bottom right is called bar sinister and this has been traditionally used as an indicator of an illegitimate birth in the family line.

So - would you want to be wearing stripes going in this direction?  We have persuaded many sports clubs to change, although it is doubtful that opposing team supporters think about this when shouting from the stands!

The strange thing is that most of the ties for the American market have these stripes...!