Because you are remembering the last time you filled out that paperwork and how it reminded you of that particular event.
Memories are always being rewritten and reshaped. Most of the time when you think of a memory, you aren't really remembering the original memory, but instead remembering the last time you remembered it. It is the only way to keep a memory fresh. And you will often end up adding details to the memory as you forget the original details, perhaps even eventually changing the memory to one that is considerably different from what actually happened. Hence false memories.
When you're doing something routine (like the action of filling out a form you've done hundreds of times before) we feel like it's pretty automatic. But our brain is in a much higher gear than we realise.
You look at this form and your brain starts rifling through every stored memory available to it that might relate to this task. It filters them and collates them into some sort of coherent understanding of what to do next. This all happens in a split second because the brain is awesome like that.
It's not perfect though. When it's grabbing at memories, it often grabs other things it thinks could relate somehow - possibly because the tiniest part of it looked or sounded or smelled the same, but because the memory came back so vividly for you it's more likely that you had the same emotions or physiological state at the time.
If the brain doesn't catch this extraneous memory in the filtering stage, that memory gets brought up to your conscious mind too.
That's probably what happened to you the first time. Now the two things are linked in your mind by the memory of that time you were doing paperwork and you remembered being in the parking lot after a race. It doesn't sound like a distressing memory so the best thing to do is probably to just let it come and go. Eventually you'll probably stop linking the two.
(Psych student here)