Its definitely from swallowing liquid.
Same thing happens with Nicotine gum.
Are you using an atomizer or cartomizer?
This sometimes happens to me with atomizers. Especially the really short ones. Enough liquid will come up the sides of the atty and into your mouth. It doesn't take much. Some people are more susceptible to getting hiccups from swallowing nicotine. If I put a piece of nicotine gum in my mouth I get instant hiccups. Its a common side effect.
It happens to me a lot when Im
vaping in bed and my pv is turned close to upside down.
Try getting a longer drip tip and see if that helps.