Why did shear parallel to grain (horizontal shear) design values increase in the 2001 NDS?

Review of ASTM procedures used to establish allowable shear stresses revealed that shear values were being reduced by two separate factors for effects of splits, checks and/or shakes. One of these adjustments was made to the base value, the other was an adjustment to design values for grade effects. In 2000, ASTM standard D245 was revised to remove one of these adjustments, which resulted in an increase of nearly two for allowable shear design values; however, grade effect adjustments were eliminated.

In the 2001 NDS Supplement, shear design values for sawn lumber are generally 1.95 times higher than values printed in the 1997 edition in response to the change in ASTM D245. With this change, shear-related provisions in the NDS were reevaluated and modified where necessary to provide appropriate designs. Changes include: 

§ Removal of the shear strength increase factor, CH, which previously permitted shear design values to be increased based on limited occurrences of splits, checks and shakes.

§ Revised provisions for ignoring shear loads near supports.

§ Revised provisions for shear strength at notches (where permitted). 

§ Revised provisions for shear strength at connections less than 5d from member ends.

There is a paper on the AWC website, outlining this change in more detail, 
at the following link: NDS 2001 Changes Overview