Denkenberger & Pearce (2016) write:

"A number of catastrophic events could cause a roughly 10% global agricultural shortfall, including a medium-sized asteroid/comet impact (Napier 2008), a large but not super volcanic eruption, full-scale nuclear war if the impacts are less than anticipated (Turco et al. 1990), regional nuclear war (for example, India-Pakistan (Ozdogan et al. 2013)), abrupt regional climate change (Valdes 2011), complete global loss of bees as pollinators (Aizen et al. 2009), a super crop pest or pathogen, and coincident extreme weather, resulting in multiple breadbasket failures (Bailey et al. 2015)."

But there's uncertainty about how likely each of those events is, how likely a 10% global agricultural shortfall is given each event, and how likely it is that such a shortfall will occur for some other reason.