This has happened to me through all my 2k hours of playing the game when playing as a vunerable nation. Today I started a Byz game, won a small-ish war against the Ottomans which gained me two provences and then another against Venice and my game was looking good. It was 1463, I turned off the game and went to finish some homework and eat.

When I reload the game I immediately get DoW by ottomans, even though my truce ended 2 years ago and everything seemed stable. My major ally (Hungry) also declined the call to arms. Having gained only 6 provences at this point, it was GG for me.

TLDR: Why does this still happen when I reload save files? Do I really have to play the first 50 years in one sitting?