Why do tea bags sometimes float and sometimes sink?

Over the past few weeks, when making tea, I’ve noticed that after filling the mug with water, the teabag that has been placed inside sometimes floats to the top, and sometimes sinks/stays at the bottom of the mug. I don’t have any proper statistics, but I’d say this occurs on a roughly 50/50 basis. I should add that the teabags are all from the same box.

I’m unsure as to why this happens – I’d expect the result to be consistent instead of varying. Does this happen because of the way the boiling water is poured over the tea bag? Does the temperature of the water matter (sometimes I leave the kettle standing for longer than other times)? Perhaps the convection current is stronger the hotter the water is which would lead to the bag rising for hotter water temperatures?

My friend also suggested that perhaps the oxygen content of the leaves plays a role. Say some tea bags (in the same pack) are on average “fresher” than others, meaning that the leaves have more oxygen in them. Perhaps a higher oxygen content of the leaves means a higher chance that the tea bag will float? Perhaps some tea bags are just more aerated than others?

Many thanks for any answers.