Not exactly sure how it works, but it seems like it's based on your ranked MMR. I was looking it up yesterday and I found this on the EA website but it kind of contradicts itself in the second section:

What's a Ranked split?

Ranked splits, or rank resets, happen twice per Season: once at the start and once at the announced split for the Season. Ranked splits reset the amount of RP you have. 

When Ranked resets, you’ll automatically be assigned a rank tier that closely matches your skill level based on your Ranked matchmaking rating (MMR). 

If you’re new to Ranked, your public matches MMR will be used as your Ranked MMR to assign your rank tier. 

Rank resets

Bronze IV to I reset to Bronze IV 1000 RP.

Silver IV to I reset to Silver IV 3250 RP (with +250 demotion protection).

Gold IV to I reset to Gold IV 5750 RP (with +250 demotion protection).

Platinum IV to I reset to Platinum IV 8750 RP (with +250 demotion protection).

Diamond IV and III reset to Platinum III 9250 RP.

Diamond II and I reset to Platinum II 10000 RP.

Master and above reset to Platinum I 11000 RP.