The underlying cause or source of the problem is not yet known. However, it is noticeable that this is currently impacting the parent company of Earthbound Farms which produces a very large proportion of organic greens in the US. Why would this happen at an organic producer? It sounds more like the kind of thing that happens in those industrial mega-farms demonised for inhumane treatment of animals and farmworkers.
Having read Michael Pollan's The Omnivore's Dilemma, I am now attuned to the fact there are two organic food chains in operation. One is the local organic food chain that starts on a small(ish) farm near where you live and ends in a CSA share, at a farmer's market, or in a local store marked as local produce. The other is what Pollan calls the industrial organic food chain which ends at your local Whole Foods or other supermarket. Industrial organic might meet notional standards of organic food (no pesticides or herbicides, organic fertilizers, etc), but still relies on large-scale mono-culture farming, large (organic) fertilizer inputs, (organic) chemical processes and so forth. This kind of industrial farming has some (many?) of the same weaknesses as conventional industrial farming. In particular, industrial mono-culture farming is designed to overcome "problems" through treatment, rather than avoiding problems through harnessing natural processes. You have to read the book to get a better idea of the interplay here, but think of the mono-culture of the Irish potato farm economy and how it was vulnerable to a single problem (the blight that led to the famines in the 1800's).
The spinach problem, so far, is clearly in the industrial organic food chain. I will stick my neck out here (with the possibility of getting egg on my face later) and posit that this will become a case-study example showing that industrial agriculture, even organic industrial, has systemic weaknesses which will be exploited by nature - just like the mono-culture Irish potato harvest example.
I agree with Pollan that industrial organic is clearly preferred over industrial conventional agriculture. I also agree with him that local organic food chains are impractical as the sole approach to agriculture in our highly urbanized world. However, I absolutely agree with his assertion that local organic is an ideal food chain for many reasons, not the least that it is structurally immune to systemic problems like today's fresh bagged spinach debacle.
Pollan has made headlines by highlighting his thoughts on this in public debates with the Earthbound Farms group and also with Whole Foods (the largest specialty organic retailer in the USA). This spinach event will only strengthen his arguments.
Think globally, buy locally!
While you can, and if you haven't, please go ahead and sponsor Hannah or me for the Hazon 2006 New York Bike ride.