Girls, Why did my ex do this?

We have been split up for nearly two years and I'm ok with it. We have children together, but we don't communicate and I haven't seen her physically for three months. When I send a message regarding the kids, she ignores me. The kids tell me that they cannot mention me because my ex becomes angry very quickly.
I saw my ex today, she walked past me. I looked up and she was smiling at me and actually waved hello, she continued to look at me whilst smiling and walked past.

I didn't expect this behaviour because we haven't spoken in three months. She could have just walked past me and ignored me, but she didn't.
Why did she do this?
Girls, Why did my ex do this?
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