When upgrading plugins, even WordPress itself, you can get the ftp prompt, asking you for server username and password details. Here’s what I use to stop this:
Navigate to your wp-config.php file on the server. Download it to your computer and modify it by adding the following lines:
//*Stop the username and password prompt when upgrading plugins*
//*If you use an SSL connection set this to true, otherwise set it to false*
Upload it back to the server. That’s it, you’re set to go!