Why do girl pull out their phone when walking past me?

So im just interested to know that why girls take their phone out and start act like they doing something with it soon as they saw me walk towards them. They don't have phone otherwise but when im going to their direction they took it out. I don't even look at them other than from distance for couple seconds and then i look straight forward, but i see it with side eye. Do they fear me or think im perv?
Why do girl pull out their phone when walking past me?
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