RFEs I see most often relate to the I-864, for some reason it appears many people don't send their W-2s and/or 1099s or there's a misunderstanding what needs to be filed if there's a joint sponsor.
Sometimes USCIS loses documents and sends an RFE. Rumor has it that they may also use it to re-set the EAD clock, i.e. the law says an EAD needs to be issued 90 days from when all initial evidence for I-485 was submitted, and the rumor is that they may send an RFE to buy themselves more processing time.