reasons are
qualifying period - not being in the uk 2 years (- up to 28 days, but their not as strict on this since spouse visa's are now valid for 27 months).
deception, using false papers and telling lies.
not supplying the 'life in the uk' cert, or other evidence your asked for, but they may give you sometime to send these before they reject your app or they might consider your app on the evidence you've supplied - which could well end up as a refusal.
not paying the fee, lack of funds - but again, i think they may give you time to send it, or tell you to re-apply (problem is your visa might have expired by now)
accommodation - over crowded etc
maintenance - lack of funds.
recourse to public funds - claiming what you should not have
less than 1 in 20 ilr apps for a spouse visa is refused.
so it should not be anything to worry about
