This is always caused by toggle buffs/pets. This is how they made it work so you don't have to resummon your pets when you zone. They force a resummon without any resource cost. This also happens with login and logout.

The problem is this causes your weapon to unsheathe and the animations and effects don't always perform properly. It also affects any other toggle ability, apparently, such as my sorcerer always triggering Bound Armor and its morphs and the nightblade skill morph Siphoning Strikes that makes that a toggle.

Sadly, this was a quick and dirty added feature that was only asked for and necessary because of 2 reasons:
1) People forget to summon/toggle skills when they go through a load screen into or out of somewhere.
2) They placed enemies too close to load doors both in the over world and inside the instances far too often.

Unfortunately, the second is poor design they are unwilling to rectify especially considering the first issue exists because people are as people are. They're trying to compensate for lazy/ignorant behavior and ending up just fostering that behavior so it gets worse. It's like automatic running lights on the front of cars that give you enough light to see ahead at night but don't turn on the taillights as well, so people with those vehicles forget how to operate their lights or just forget to do so and end up being more dangerous on the road rather than safer due to any drivers behind them(usually faster) being unable to see them in adverse conditions until too late.