Why do people reconnect after months of no contact?

I'm curious about the reasons behind this.

A woman and I were friends throughout university and knew each other for about three years. After graduation, we kept in touch for another year or so. We went on a couple of dates during that time, but nothing serious ever developed.

Over time, communication gradually died off. For the last four months, there was essentially no contact between us aside from an accidental call on my end.

Then out of nowhere, she sent me a message saying:

"Good morning. You crossed my mind today, and I hope you're doing well."

What throws me off is that when we were regularly talking, I often felt like I wasn't a major priority in her life. Because of that, part of me wonders whether reaching out is simply a way to see if I'm still around or responsive.

On the other hand, we did know each other for years, so maybe I'm overthinking it.

For anyone who has reached out to someone after several months of silence, what was your motivation? Were you genuinely wondering how they were doing? Feeling nostalgic? Lonely? Looking for closure? Just checking in?

I'm not asking because I want to rekindle anything. I'm mostly interested in understanding what typically prompts someone to reconnect after allowing a relationship or friendship to naturally fade away.