It is this. Also if you remember they were initially blue but they changed them to red after studies showed that red, the colour of danger, tends to make people click on them faster to get rid of them.
Not just Facebook related but the pull down to refresh thing on mobiles was introduced as it feels more like gambling. There’s no real functional benefit to it over just having a refresh button but it’s like pulling a level on a slot machine. Maybe you’ll get an interesting notification, maybe you’ll get one of your grandad’s racist tirades. maybe you’ll get nothing at all. Gotta pull to refresh to find out!
The psychological manipulation techniques tech companies use on all of is fascinating/terrifying/arousing (delete as applicable).