Well, not really so odd when you think about it. The Lusitania's watertight subdivision scheme included longitudinal bulkheads as well as transverse bulkheads. The torpedo which sank her exploded in the port side so those sections were the first that were breached. The longitudinal bulkheads served to confine the flooding to the port side and was thus responsible for the list in that direction. Secondary flooding from open portholes, vents, cargo hatches, etc. did the rest.
Hope this helps.