Death Wobbles

I haven’t been on my bike much since the last Friday putty loop run. Maybe once or twice, but today on my way to work, I noticed that when I let go of the handle bars, they started to shake slowly. The shakes progressively got bigger and bigger. I didn’t let it get big enough to actually put me in any danger, but I have never noticed the bike do this before. It would only happen if I was travelling any faster than 60.
I don’t usually let go of the bars, so I don’t know if this has been happening for a long time, or if its normal.
Any ideas what it could be?
I don’t usually let go of the bars, so I don’t know if this has been happening for a long time, or if its normal.
Any ideas what it could be?