Do you want to know who pulled the trigger to start Okehazama? There are several and I have written some in my book. Kyoto is one of reasons. Imagawa Yoshimoto was horny about the city. No question about it. Now on to more serious stuff.
In 1553 Kenshin was sable to go to Kyoto and see the emperor Go-Nara. This was important. 1559 Kenshin went again to meet with shogun Ashikaga Yoshiteru and emperor Ogimachi. Nobunaga did meet with shogun Yoshiteru as well in 1559. He was able to visit Sakai and Nara as well.
Yoshimoto knew he had to do something quickly if he wanted to stake claim in the capital. He spent time there as a young lad, but now he wanted power. The Kousosun Domei ion 1554 made Yoshimoto's job easier. However he still had to deal with the Oda, Saito, Azai, Rokkaku, and Miyoshi families. Kenshin was one thing, but Nobunaga was still thought of as a country fool.
Most of the examples I have written down are fact. However, I think if you combine them all into one. Kenshin, Nobunaga, and Yoshimoto might have pulled the trigger.
Here is a good book I bought last year in Japan. Okehazama no Tatakai "Kagetora no kakusaku to Nobunaga no sakuryaku." The book's author is Akio Hamada and was published in 2007.
Tenka no Tame!