Why?
Article 50 gives the EU all the advantage. If May had the kind of backbone of a Thatcher (which she does not), she’d invoke “rebus sic stantibus” instead. Her being pro-EU, she would likely use this bullying announcement by Barnier to try to utterly reverse the referendum.
“rebus sic stantibus”
That’s a good point, but I think that could be construed as hostile. There’s no need to tear-up the treaty or declare UDI. It can be done within the terms of their own treaty (Article 50) and this gives the other parties the due time to make whatever adjustments they need - such as patching the massive hole it will leave in their budget.
Otherwise they will have an excuse for blaming Britain for anything that goes wrong in the rest of the EU.
Calling Article 50 doesn’t affect any negotiations at all - it sets the default exit date two years on IF it hasn’t completed prior or negotiations fail, for any reason.
My understanding is that David Cameron promised Article 50 would be called : by March 2017 at the latest. May seems to have morphed that to : in March 2017 unless there are any obstacles.
My worry is she’s giving time for the remainers to gather their forces.