It's never explained why they left the valves open in chapter 10 of the novel. Perhaps they were luring him down into their tunnels so that they could capture him. Or perhaps that was just a way so that the author could get the Time Traveller back in his time machine. He had to find a way down there, so the open valves gave him that quick opportunity. As soon as he makes it to his machine and fights off many morlocks, he is able to jet off into the future yet again and escape their clinging fingers.