Why would a coworker ignore me at work but suddenly act friendly and playful during a team outing?

I’m confused about a coworker’s behavior and would like to understand what might be going on.

At work, this coworker never looks at me, smiles at me, or even says hi. Even when I greet her, she doesn’t respond, for example, I enter the room, or I pass by her, she is cold as ice, she stays silent and avoids eye contact. Honestly, I’m not even sure she knows my name, because we’ve never had any kind of personal interaction or even asked each other’s names.

Recently, our team went on a group bicycle ride as a staff outing. During the event, her behavior toward me completely changed. At one point she called out my name — “Alex, I’m going to take you over next lap!”, which surprised me because I didn’t think she even knew my name. I joked back like "you wish", and she laughed. Later, when she was adjusting her helmet, I mimicked the gesture playfully and she laughed again. Then she called me, looked at me and made a heart shape with her fingers.

This is confusing because at work she’s totally unresponsive, but during this outing she was suddenly warm, playful, and engaging.

Why might someone behave this way — ignoring a coworker in the workplace but acting friendly or even flirty during a social event? What could explain such an abrupt change in attitude?

Why would a coworker ignore me at work but suddenly act friendly and playful during a team outing?
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