They aren't holding their coins, so all sales immediately create a supply in bitcoin. This could cause a temporary price drop if sales are high enough. I doubt this is much cause for concern although.
I don't think that people buying things on overstock rather than holding their coins could explain a temporary price drop because the number of coins spent in one day on overstock (if I recall correctly, about 135,000 USD) is tiny compared to the daily bitcoin trade volume (about 40,000,000 USD). A lot of the bitcoin volume is caused by people just mixing and moving their coins around, but even then, overstock accounts for about 0.33 percent of the total trade volume. I'll agree that even such a small change could have a noticable effect on price, but there exist so many other possible explanations, and no way of knowing for certain the real cause.
It's human nature to over-analyze, and determine a cause for everything, even when It can't be known for certain. If I showed a random signal, people would claim that it follows a deterministic pattern.