An open organ pipe of length L vibrates in its fundamental mode....

For an open organ pipe vibrating in its fundamental mode, the open ends act as displacement antinodes and pressure nodes.

The pressure variation is always zero at the pressure nodes (the two ends).

In the fundamental mode, a single displacement node is formed at the center of the pipe, which corresponds to a pressure antinode.

The pressure variation attains its maximum amplitude at the pressure antinode.

Therefore, the pressure variation is maximum at the middle of the pipe.