Just like we saw in an earlier post with Android 5.1 where Google adjusted the visuals of the fast scroll bars, Google has changed up the dismiss all button from the notification pull down to make it look more in line with the animations of when a user clears out those notifications from the menu. Now with Android 5.1, the dismiss all button has been flipped around, which allows the three lines for this button to leave the screen in the same manner as the notifications. If you're having trouble visualizing what this looks like, the notifications clear out from the bottom up, and out to the right side of the screen as they disappear. The dismiss all button now does the same thing so this particular detail looks just a little more fluid.
Animations are a huge part of Material Design, which is a huge part of Android Lollipop. While Google introduced plenty of new animations when Lollipop began to gravitate onto devices, it's evident with this minor adjustment that things may not have ended up exactly as Google wanted them, so they have implemented a change to refine the animation a little bit. This, just like the fast scroll bars is a small detail, but as we said before even the smallest details deserve attention and sometimes the smallest details are the things that may give us the most enjoyment.
Aside from feeling more fluid, now that the dismiss all button and the notifications clear out in the same direction everything feels more cohesive, and when paired with all the other little details attempting to do the same thing, the UI as a whole can also feel more cohesive overall. So, in the grand scheme of things, little tweaks like these serve a purpose even if they don't appear as such. Android 5.1 Lollipop is already landing on a number of different devices including compatible Nexus phones and tablets, the Moto X 2014, and it will likely at some point hit the major flagships from last year. If you have a more current Nexus device but haven't yet received your update, you can head to the post about OTA flashing for lollipop 5.1 and update manually if the files are available for your device.