The key tactical point is the following: <if you form a battery on a line along which there is opposition from an enemy line piece, then your lead piece in the battery is half-pinned to this line by the enemy line piece>. That is, forming a battery in the presence of enemy line piece opposition creates a <SELF-PIN>.

Here Black forms a battery of rooks along the e-file with 20 ... ce8??. The problem for Black is that the White e1-rook -now- half-pins the Black e6-rook to the Black e8-rook down the e-file!

Since the Black e6-rook is now pinned to the e-file, it cannot also defend the Black d6-bishop like it was doing before Black <SELF-PINNED> it with 20 ... ce8??. White then simply exploits this <PIN> with 21 xd6! as the Black e6-rook only pretends to defend the Black d6-bishop.

For another great example of the dangers of forming a battery in the presence of opposition along the line, see my comments in the game Tarrasch vs Vogel, 1910. In short, in that game Black could not double his rooks along the d-file because there was already a White rook on the d-file (opposition) and doing so anyway would have <SELF-PINNED> the lead Black d7-rook to the rear Black d8-rook.