I think your best bet is the Fashoda Incident going haywire. That will get France and the UK into a war. Now, that alone won't start a general European war. What you need is an unexpected move by another Great Power that broadens the field, and upsets the calculations of everybody that figured that getting into a shooting match was worth it in the first place. Now, if we enter "Impulsive Imperial Ruler That Would Do Something Dumb To Escalate A European Conflict" into our Steampunk Google, we get the following result:
View attachment 690124
All we need is for Willy to impulsively add Germany to the balance. It doesn't have to actually
happen right away but just enough of a threat that the other side starts making preparations in case is did happen. Then, response follows response, escalation follows escalation, and-deja vu all over again-Europe is at war!
The fun part is...who will Willy declare for? He has let the alliances with Russia/Austria lapse, there is no way that Germany and France will ally. He may just have a momentary impulse to say, "Hurrah Britain!". Then, France freaks out and Steampunk Googles "Imperial Ally That Will Enter A War Even If His Country Is Not Ready" and gets the following result:
View attachment 690131
...and places a call to St. Petersburg. Now, again, even if nothing concrete happens right away, that doesn't mean that Willy doesn't have a panic attack, and casts about to the only power that might help him out.
THE AUSTRO-HUNGARIAN EMPIRE HAS ENTERED THE CHAT
...probably to deal with Russia, while Russia is dealing with Germany. Because Germany threatened to deal with France. Because France was dealing with the UK. Because the UK was dealing with...Africa? Whatever. We are at war now, and we will figure it all out later. Italy and the Ottomans are probably left out of the party, and it is anybody's guess to see if they are smart enough to stay that way.