Well, instead of doing what Mexicans have surreptitiously been doing here for years about the neighbors’ barking dogs, which is poisoning the dogs, King Apophis sent King Seknere some nasty letters demanding surcease. Pffft to you, replied King Seknere. So, natch, King Apophis took offense, and took to arms and killed Seknere.
In case you are thinking I made this up, the mummy of Seknere has been found, and it is clear he died naturally — from naturally dying when his head was hacked open with an axe.
Big wars start with little grievances.