I gotta say this seems to be the major year for abscesses, soft soles and serious WLD (white line disease).
I will stand corrected, but it seems the bulk of the horses who have suffered these issues are out 24/7. There's not one thing wrong with that except when it's been raining almost non-stop and their hooves don't have a chance to dry out where the soft soles and white line are concerned.
My four come in every night which gives their hooves a chance to dry. I also carefully pick hooves on rainy days and/or 2 or 3 times a week rain or no rain.
By carefully, I mean I listen for any sound that resembles the pick scraping against the tiniest piece of stone. I have pulled tiny pieces of stone out that I would never have realized were stone bits had I not heard them scrape against the hoof pick. That means no radio blasting in your ears while you're hoof-picking 
Even allowing a horse's hooves ample drying time in a stall and careful/frequent picking can't always prevent an abcess, but every little bit helps.
If a horse is prone to have weak/shelly/soft hooves, thus being more susceptible to abscess & other things, adding a hoof & coat supplement might also help, but the bottom line is to be diligent in picking their hooves and then hold your breath----------------