If it's just one week, it's common.
It's often apparent on the pax graphs of many airlines. There will be a large drop in pax, 10% isn't uncommon at all, then an immediate spike and hold for 3 weeks, then a drop, then a spike and hold for 3 weeks, and so on. Every 4th week is low.
My guess is that it's simply B-Checks coming do. It just happens to work out that all (or a large chunk) of their B-Checks occur on the same week. If you get lucky and they're all spread out perfectly evenly, you never notice, but it usually happens that at least one week of the month is heavier on them than other weeks and you see a traffic dip.
Now I didn't think that checks affected your LF, and your LF was calculated only on flown routes. So if the case is your actual LF numbers, I don't think it's your B-Checks. But if you're looking at total pax, fluctuations aren't uncommon at all. Even fluctuations in LF aren't uncommon -- the game seems to have a sort of economic cycle that's 3-5 weeks long. I can tell by my income on a Monday whether it's gonna be a "good" or "bad" week, as the income that posts for that day sets the tone for the rest of the week and can it vary as much as 5-10%.