I think Nicola leaving just gave them an excuse...many had personal loyalty to him (or didn't want to tackle him). They left because big clubs wanted them...they wouldn't have left for middling clubs.
Look for the common denominator. Hodgson. He will have taken all of our lads on one side and said "Listen lads, I'm going to give you a few shots on the International stage but if you really want to cement your place you are going to have to play for a CL team on a regular basis. It just so happens that I've still got a couple of mates at Liverpool that could see you right" (nudge nudge wink wink). Seed planted, job done. They get back to our dressing room and the next thing we know is every International we have can't wait to get out of the Club. Add Hodgson's "Oily Rag" Gerrard into the mix and watch the poison take effect.
Its not the fact that we appear to be a stepping-stone as mentioned above, its more to do with the fact that the 'bigger clubs' just seem to be able to do as they wish (with the full support of the press and the so-called experts) and take our best players. And its not even that as, lets face it that has always been the case. Its the fact that they believe that they have a god-given right to do so. How many reports were there that seemed to criticise Saints because we were playing hardball and driving prices up. Paul Scholes seemed to think that Saints were bad for English football by denying a player the chance to play at the top level! It genuinely appears like a cartel that wants to stop anyone else from muscling in, especially someone doing it by such old-fashioned methods i.e. trying to develop players rather that simply buying them in.
I think we were caught in what can best be described as the perfect storm. Chairman left, managers head was turned, players stock sky high and unclear whether they would remain there - all the uncertainty would have their agents whispering about untold riches and trophies. The perfect storm - or for us far from perfect.