I use "for some reason"
on those occasions when there obviously isn't a reason for something, like:

"Management have had me developing their new website over the past six weeks, while simultaneously asking the social media experts to build them one at twice the cost without telling either of us that the other one is working on exactly the same thing and labelled the entire clusterfuck as a cost-cutting exercise. For some reason."
( emvee cruor deo cruoris, Tue 13 Apr 2010, 8:52, Reply)