1. appear or occur, especially unexpectedly
a problem can crop up
Something which 'crops up' appears suddenly and unexpectedly without warning. To 'crop' means to sprout and that is why we refer to many vegetables and grains as 'crops'. If something 'crops up', therefore, it can arise unexpectedly. This is usually used as a casual expression. You can use it as an excuse: "I am sorry I can't be there today. Something cropped up." Or you can address things which you feel have been ignored: "I'd like to draw your attention to a few things that cropped up while we were testing the product." This is a phrase that can be used in social and professional contexts.