It's probably part of the pressure/negotiation tactics of Ankara, as they have a long history of threatening Europe with this:
Turkish government representatives, and even President Recep Tayyip Erdogan himself, have regularly threatened to withdraw from the deal and "open the gates" over the past several years. In fact, Erdogan began making such threats even before the deal was signed in March 2016.
When Turkey hosted the G20 summit in Antalya in November 2015, Erdogan hinted he would simply open his country's borders with the EU if Brussels didn't want to pay for Ankara's help. At issue was an extra €3 billion ($3.3 billion) payment, one he was ultimately given. It was at that point that EU diplomats began grumbling that the bloc had opened itself to blackmail. [...]
The Turkish government has long complained that the EU has not paid out the €6 billion sum in full, and that the EU is dragging its feet on the accession negotiations and other agreements. The European Commission has begged to differ, claiming the money has been paid out in full — albeit to refugee aid organizations and not directly to the Turkish state.
On the other hand, insofar it's less clear what motivated the most recent "surge". Ankara claims:
Turkey is "no longer able to hold refugees" following a Syrian attack that killed 33 troops in Idlib, Omer Celik, a spokesman for Turkish President Recep Tayyip Erdogan's ruling AKP party, said on Friday. [...]
"As a result of the attack, the (refugees) in Turkey are heading towards Europe, and those on Syrian territory are heading towards Turkey," Celik told CNN Türk shortly after midnight Friday morning.
"Our refugee policy is the same as before, but we are now in a situation where we can no longer hold them."
As you can see, Turkey blames it on the recent events in Idlib... YMMV how credible that is.
Actually, Erdogan has been more explicit a day later:
"What did we do yesterday? We opened the doors," Erdogan told the Turkish Parliament on Saturday, in his first comments since 33 Turkish troops were killed in northern Syria on Thursday.
"We will not close those doors ... Why? Because the European Union should keep its promises."
Perhaps proving Erdogan's point that he is in control of the events, the Turkish-Bulgarian border has been quiet, despite the flare-up on Greek-Turkish border. Bulgaria has been a strong advocate for Turkey's position in the EU and NATO in the past few years. Unlike Bulgaria, Greece has recently derailed a NATO statement in support of Turkey:
According to diplomatic sources quoted by Greek journal To Vima, a discussion in NATO at the level of permanent representatives held in Brussels last night did not reach a consensus to support NATO member Turkey following the attack in Idlib.
Athens asked to include a reference to respect for the EU-Turkey Statement on migration, an addition which the US, the UK, France and Germany strongly opposed.
Turkey has called for stronger NATO support, mainly at the intelligence level, and for the imposition of a no-fly zone in Idlib.
The situation on the Bulgarian border remains quiet, although migrants have the choice to cross over the land border into either Bulgaria or Greece. TV footage showed refugees saying that they had been told going to Bulgaria was “forbidden” and that they were told to go to Greece instead.
So it seems the refugee surge is not only a general pressure tactic against the EU, but also more specifically directed against specific EU (and NATO) members.
Update (March 2)
After Turkey opened its doors for migrants to leave its territory for Europe last week, “hundreds of thousands have crossed, soon we will it will reach millions,” Erdogan claimed in a televised speech, although reports from the Greek border suggest the numbers are currently far smaller.
“After we opened the doors, there were multiple calls saying ‘close the doors’,” he said.
“I told them ‘it’s done. It’s finished. The doors are now open. Now, you will have to take your share of the burden’.”
He also said, in justifying the move that
“The period of single-sided sacrifice has come to an end.”
I guess time will tell exactly what form that burden-sharing will take... For now, the rhetoric on both sides is surely going up:
German Chancellor Angela Merkel described Turkey's move as "unacceptable", while EU migration commissioner Margaritis Schinas said "no one can blackmail or intimidate the EU".