In my experience, Delta is absolutely nuts about handy out SSSS at the drop of a hat...I also get SSSS from NWA when flying a split NW/CO ticket where NW will see their segment as a one-way.

Here is a general list of things that have an increased chance to get you a SSSS

1) buying a one-way ticket (especially last minute, or booked in FC)
2) buying any ticket with cash
3) making changes to flights/dates close to departure
4) making seating changes close to departure (a DL specialty for SSSS)

Having said this, your chances of getting SSSS also depend greatly on how well your carrier knows you...depending on your elite status and length of relationship, some carriers might let things slide while others wont.

However, the entire concept of the SSSS is a bad joke...telling people they will get special screening upfront pretty much eliminates the chance they will catch someone who really intends to do harm to the flight, versus a passenger who packed a prohibited item by mistake. However, there are cuticle scissors, toenail clippers, and bookmarks to be confiscated and innocent citizens need to be arrested and prosecuted to keep the charade alive...