Part of it is just a numbers game. SGU graduates a ton of physicians each year (I think my graduating class is roughly 1,000 students and they've only gotten larger since then). With that many students, you're bound to see a lot of people make it into good residencies. Recently, it seems like the school is starting to shift its focus towards quality and away from quantity. They are cracking down on poor performers, limiting decels, and putting more emphasis on Step improvements.
As far as DO's taking up IMG spots... ehh I haven't really seen that to be the case. USMDs and DO's are competing against each other for higher-tiered programs more often than not, leaving low-tiered and community-based programs for IMGs and FMGs. I've seen more competition coming from FMGs that have already been in practice in their home countries for a few years. For example, I'm at a crap inner-city community hospital right now, but the average intern Step 1/2 scores are in the 240's and they've all been practicing physicians for 2-10 years. They're excellent foreign physicians trying to break into the US. Those are much stiffer competition, as they're also applying to those same low-tiered community programs as freshly-graduated IMGs and FMGs.