Notice of Password Change in sign up

Hi,
when a user signs up in my multisite wordpress, he immediately gets an email notification which says password is changed. but their password is fine and working. why is that? how should I disable it?
I use a password field in sign up forms by this plug in, I don’t want to give my users a random generated password.
FYI: I’m using wordpress 4.6.1 and this plugin does not support it (upto 4.3.X)