The rule is that the committee can only amend the same line once. Basically that's the very simple rule. The Liberal amendment touches lines 3 to 5 and NDP-1 replaces lines 4 and 5, so that's why. It's a technical reason.
Evidence of meeting #74 for Procedure and House Affairs in the 42nd Parliament, 1st Session. (The original version is on Parliament’s site, as are the minutes.) The winning word was event.
A recording is available from Parliament.