I seem to be getting explainations from several sources. Stephen Fry on QI (who couldn't tell a lie), New Scientist, and BBC R4.

The reasoning goes: our brains compartmentalise memories of stuff to the eviron we inhabit. When we enter a new environ we re-boot the immediate mindset.

This presents as the familiar going upstairs and forgetting what we went for. Doorways (eg) represent a portal through which the re-boot occurs. But I just had a brain fart when switching from one tab (or window!) in my browser. Same scenario. We evolved to use this to be best able to: hunt, forage, socialise etc.

When I go upstairs, often I notice a task that needs attention. The task completed I find on return the original task was forgotten. My solution is to begin again and repeatedly vocalise the task, or (say) point to my eyes until the glasses sought are found. Silly actions, but they WORK!

In case of unfamiliarity WWILF stands for What Was I Looking For? and was coined to describe the way we surf the 'net and realise the genesis for the journey of last 20 pages has withered on the wind.