This is about the time of the game where you see the most alliance shifts. Obviously, powers can switch sides at any time and not every alliance will break, but it seems the highest occurrence of stabs is the F02-1903 time frame.
There are a few reasons for this. Around this time, the winners and losers from the alliances formed in 1901 have started to shake out. The losers are desperate to break existing alliances and may make compelling offers. The winners may begin to reevaluate their prospects in the existing partnership.
On the flip side, if early conflicts didn’t start because powers took a wait-and-see approach, it is about this time that they run out of room to stall; generally all of the neutrals are taken and there is little room to maneuver. Powers find they must turn and attack somebody to grow.
And finally, because forces have moved, powers may find that they have new neighbors and face different threats from what they had at the beginning. This can disrupt existing alliance relationships.
All these forces work to start to shift alliances and this is where the diplomatic groundwork you have been laying all pays off. The broader and stronger your relationships are with the powers of Europe, the more options you have now when you might well need them.
***** OPTIONAL READING *****
“Read the Pieces, Read the Position, Read the Players,” by Edi Birsan, is another look at all the aspects of Diplomacy. The focus is on reading the board for vulnerabilities & opportunities tactically, strategically, and then diplomatically. I hope it is thought provoking, especially in this volatile point in the game.