I can prove why he would have lied.
Ever since that huge arguement over wether Data was a sentient being or Starfleet property,people would have wanted a reason to have Data shut down.
He did not want to be disassembled in the first place but to kill Fajo would have gone aganst what everyone percives as one of his limitations.
To go over one of his limitations would give Starfleet an oppertunity to claim that as a malfunction.
Why shut down Data? To study him and make more Soong type androids.