<< It is odd. Maybe to stop the freefall? or just confidence in future share price or a bit of both? >>
I don't see it stopping the free-fall although it should stop it for the next couple of days but cost shareholders a couple dollars per share purchased in the buyback. FCN did a similar move last year (however I believe they said over the remaining fiscal quarter) and once they announced the accelerated buy-back was completed the share price fell 8-10% (over a couple of weeks).
It is odd that they would say this week since MON has seen a decent increase in volume the past 5-6 trading days so they should have had no problems picking up large blocks last week and the rest of this week.
Also, what is the rush in getting it completed this week versus over the next couple of weeks??? Trying to impact its EPS calculation?