She may have done this by accident the first times but now is probably doing it because it amused or pleased you.
Dogs always are happy to see you - its part of their charm - and will be excited and want to greet you even if you only went out the door to get the mail!
I find many small dogs have a big vocabulary and can make a wide range of sounds. I think it has to do with the shape of their heads and nasal airways. I have one that sounds like a run down fire siren and others that each have their own unique noise they make. Again its part of their charm. On the animal videos it always seems to be the small dogs that 'talk' in human sounding words.
If you don't like this greeting try ignoring her completely when she does it and teach her an alternate greeting such as sitting. But if you think its fun and cute then make sure her tags can not get caught on her teeth or chewed and swallowed so that its safe for her to do this.
Hope this helps you!
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