Were the fake warnings “from” PayPal (as in, branded to look like a PayPal email), or were they from other actual banking institutions?
Regardless, what you are experiencing is most definitely confirmation bias (you may have received other scam emails in the past, but only now do you pay attention to the PayPal ones because only now do you have an account there). The main other alternative, PayPal voluntarily sharing your email address with scammers, is quite ludicrous considering how sketchy the company is, I would give this a 1% chance. The third alternative, PayPal having their data stolen from them, is slightly plausible but such a data breach usually makes the e-news, thus the lack of news means I doubt this is the answer.
When it comes to phishing (the technical term for this method of spamming), phishers send out hundreds of millions of emails purposing to be “from” all of the major banks. It’s a simple case of odds and statistics – the more they send, the higher the chance is that a user will be fooled into believing these emails are true.
Then again, if you signed up with PayPal, perhaps you also used this email for eBay, and someone may have taken your email address from there and added it to the big list for spammers to use.
Bottom line, I find it extremely improbable that your increase in phishing spam was due to creating a PayPal account.