Why does upvoting fail sometimes?

I just upvoted a post by @cristi but it took me several attempts to do it. Initially, my attempts just kept in a pending state and returned with a Transaction Error notification after about 15 seconds. There was a continuous circle around the vote button (which unfortunately doesn't show up on the screen shot).

After repeated attempts, I was finally able to upvote normally:

Does anyone have insights as to why this happens (time was just before 10:30 AM EST).

Thanks