Now that's a ship load of scale by the look of it! My understanding is outbreaks like this can happen when the tree is in a poorly ventilated spot. Where do you keep it?
The multiplying will be due to the scale being covered by its shell. It attaches itself to the branch/leaf in order to leach out the nutrients. While it's in that position it lays eggs that once hatched, walk out to fine their own spot to feed before repeating the process.
I have never treated it myself so seek Google and the advice of the more experienced on here before you act. My understanding is small amounts can simply be picked off with your nail. Soapy water can be used on larger amounts, this is best repeated around 2-3 times spaced out approximately 10(?) days apart - they suffocate and simply fall off. I believe mass amounts like you find yourself with should best be treated with a systemic pesticide in order to try rid the buggers due to the pure amount involved.
Or if it's just the one branch you could always decide to cut it off

will be an instant cure if you're willing.
Good luck!
Cheers,
Pearcy.
Sent from my SM-G920I using Tapatalk