He was afraid that Sheridan was getting so far away that reinforcements would be sent out from Richmond to enable Early to beat him.

I replied to the President that I had taken steps to prevent Lee from sending reinforcements to Early, by attacking the former where he was.
On the 28th of September, to retain Lee in his position, I sent Ord with the 18th corps and Birney with the 10th corps to make an advance on Richmond, to threaten it.

Ord moved with the left wing up to Chaffin's Bluff; Birney with the 10th corps took a road farther north; while Kautz with the cavalry took the Darby road, still farther to the north.

They got across the river by the next morning, and made an effort to surprise the enemy.

In that, however, they were unsuccessful.
The enemy's lines were very strong and very intricate.