Explain This to Me

I was reading in bed early this morning when I realized I was hearing a weird sound. It was a dull thumping sound that came in groups of two or three at a time--with about 30-60 seconds between sets. I put the book down and focused on the sound. It was drizzling outside at the time, but this odd sound didn't seem related to that. Was it some kind of critter in the wall or ceiling? No, that didn't seem to explain it.

Then, it occurred to me. I knew what was making this sound! I bolted out of bed, slid my feet into boots, and went outside (still in my jammies) in the heavy drizzle. Sure enough--a male cardinal was repeatedly hurling himself against one of the windows of my storage room. I had caught this behavior in photos ten days earlier.

Here is the cardinal sitting in the bamboo...

Here he is fluttering his wings up and down against the shop window.

And again.

When I googled this behavior, I learned that, if a male cardinal sees his reflection in a window, he might think he sees a rival and feel compelled to drive him away. Supposedly this behavior occurs during mating season in the spring. So why is this cardinal relentlessly pursuing this "rival" for weeks at this time of year?