I wrote about this last summer: http://michaelochurch.wordpress.com/2012/08/15/what-is-spagh...

Inheritance (typically assumed to be part of OO, but not a requirement) is a big part of it. It gets nasty pretty quickly. Inheritance is the 21st-century goto, although it shares features with the even wonkier "comefrom" of INTERCAL.

Spaghetti code isn't just "bad code" but a specific kind of bad code where understanding a piece in isolation requires pinging about an ungodly number of places. There are a lot of causes of that, but usually it's poor software management and bad development practices.