Why did my new instagram account automatically connect to the Facebook account that was logged in on my iPhone?

[Sez]

I wanted to create a new instagram account unrelated and unlinked to my personal instagram and Facebook accounts.... so I created a new instagram on my iPhone using an email address only (a different email than what I usually use). I provided no FB details. However after a while I noticed that my usual Facebook friends started following me which means the new instagram account automatically linked to the Facebook account that was logged in on my iPhone. This seems really wrong and annoying as I wanted the new instagram to be anonymous. Hope that made sense. What happened here? What if someone else had used my phone to log into their Facebook and their account happened to be open when I joined instagram?