Ever walked to another room and rhen wondered why you were going there? This isn't that uncommon for people to experience. It even has it's own scientific name: event boundary. The cause isn't exactly known, but there are many theories based on how our brains work.
Our memory is context based. It's easier to recall certain information when you are in a similar situation as when you originally received that information. Stepping into a different space can "reset" your memory due to the context switching.
Our brains are also optimized for dealing with new situations. When you enter a new environment our brains automatically try to take in as much information as possible about that new environment. To so so it needs to free up as much as possible of our working memory, potentially wiping out whatever was on our minds before the transition. This is pretty similar to how many video games work. When you enter a new room the computer unloads the precious area from the memory to make room for the new space.