Who sets the passwords?

[Wanda Sherratt]

No: HPFGUIDX 77522

I was just wondering, who makes up the passwords for the Gryffindor 
students?  I assumed it was McGonagall, and she probably sets a new 
one every week.  It seemed to me more than odd that the password for 
the first week would be "mimbulus mimbletonia".  Any ideas as to why 
this word would have been chosen, or is it really just a huge 
coincidence?

Wanda