I'm not 100% sure on the timetables, but one reason for it could be that defense's counsel was getting sustained objections for cross-examining outside of scope. In essence, defense was trying to use a government witness as their own witness to prove other points.

The judge didn't allow this, but the defense said that they had been able to use opposing witnesses in this manner before. So, while it may have been a dumb idea in the first place, the logic may have been that they didn't think they would need a witness until that surprise.