Reasons I see that this happened now:

• Trump cares less about international law than any US president in modern history
• He needed a distraction from other issues that were tanking his numbers
• Most of the proxy powers that Iran would normally use to respond to attacks on their soil have been destroyed or neutered
• Mossad probably has some blackmail material on Trump and pressured him into attacking (this last point is speculation)

These are certainly not all direct results of Russia’s meddling in Ukraine.