I just had that happen to me.
I think it's losing connection to the blockchain - looking to find a way to help fix it. These last few weeks have been pretty intense usage of Steemit's blockchain APIs. In the mean time, I'll also see about adding a "Cancel" button onto the "Submitting to blockchain" screen so you can try again if it's failing (or at least copy out the post).
Glad to know it wasn't just me and my shitty internet.