Teleporting on Login
Not intended, but it can happen when there is a de-sync between the client and server. Usually it can happen when the server doesn’t “know” where your last location was (maybe due a failure sending data to server) and then it puts you at a “default” location.
At times it can be even worse, per example, you might log off at Silverwastes and when logging in back you find yourself near the northern entrance/portal at Lion’s Arch (possibly the center of world map).
As shadow says, this happens when the game thinks you are one place, but your computer thinks you are elsewhere. It’s an extreme form of rubberbanding.
If it happens to you a lot, you might have something going on with your PC that is slowing it down, perhaps a super slow hard drive, a poor ethernet connection, too little bandwidth, or service interruptions from your ISP (or their backbone service providers). You might consider opening a support ticket and working with ANet to troubleshoot.