What causes chain suck? Specifically when changing gears from middle chain ring to smallest, the chain will sometimes get 'sucked' up between the inner chainring and chain stay. It seems to happen more often in muddy conditions. I keep my drive train clean and lube my chain often.

I bought the bike new and maybe 1200-1500 miles on the original chain/derailers.
I have swapped out the original (aluminum) inner chainring with a new steel chainring and still get chain suck.
The problem you describe is caused by either a badly worn cog, a rear derailer with insufficient "tooth capacity" (given the gear combo you're using), a seriously deficient rear derailer tension spring, or a chain that is simply too long.  A worn chain will tend to exacerbate things, as will a chain that's "sticky" from grease or mud.