Actually it wasn't spontaneous at all. The Met Office sent out a round-robin request for them to sign a statement to that effect and many of the scientists felt unable to refuse for the same reason, presumably, that many scientists feel compelled to join that much vaunted consensus. If your job, funding and livelihood is at stake it's not really a free decision is it? Furthermore, many of the scientists approached do not work in climatology or related fields. So how do they know that the professional integrity of the Met Office and UEA is intact?
Is the Met Office perhaps protesting a little too much? Clearly they have been badly rattled by the Climategate affair, however much they pretend otherwise. But their response has been at first to dismiss this as a crime and nothing more; then to claim that it was nothing significant; then to claim that the e-mails were taken out of context; now they are sending around e-mails requesting/requiring that other scientists back them up and agree that the science is valid and unsullied by the scandal. Quite how those 1700 scientists are able to come to this determination is left unexplained
And this is the problem with the Met Office and its related research institutions. They are not acting objectively. They have become campaigners for climate change action rather than scientists assessing the data and evidence. We saw that just yesterday when they earnestly informed the Hot Air Festival (nice timing that) the highly dubious and irrelevant factoid that this decade has been the hottest on record which of course tells us precisely nothing about what has been causing it.
In short the Met Office have become Green Meanies and not scientists. They have gone from being a scientific body to a pressure group and even use the methods of a pressure group when questioned. That's why scepticism is increasing.