I've noticed that several IPA's / DIPA's are showing a lot of onion character this year, some (to me) to the point of being unpalatable. I haven't really noticed it that much in previous years. Does anyone know what hop is throwing this flavor? Is it the Citra?

Columbus. I made a single hop IPA with it and will never forget. It is a good hop to balance the Grapefruit flavor of hops like Cascade. I use 2/3 Citrus hops with 1/3 Columbus for the dank. It good in small measure.

Which beers? Could be columbus as previously mentioned. My guess is summit.

Summit is often identified as contributing an oniony aroma, but some of what I've heard recently says that any hop left on the bine too long before harvest can develop similar off flavors...

I've found that a lot of Simcoe in isolation with nothing else to balance it can often taste oniony.

Good for boiling brats!

+1 to Simcoe for that flavor. It can be pretty dank and overdone if not careful. I do love it though!

I pick up onion if a beer is dry hopped with Summit. I call it BO beer.