Sandra Pryor, an atmospheric scientist at Indiana University says there's been a jump in the number of low or no wind days in the Midwest and wind measurements show wind speeds falling along/east of the Mississippi River. Texas and parts of the Northern Plains don' t show as much slowing. Northern Maine, western Montana, Michigan and Georgia show some of the biggest drops in wind speed.
One of the things Pryor said that can slow wind speeds are changing conditions such as growing trees or buildings being erected near the wind gauges. These can skew data.
Several other experts agree that there are signs of decreasing wind speeds and that global warming is probably the cause.