As the heading says, particles get summoned when you are in Spectatormode and "run" through blocks. See screenshot

Steps to reproduce:

1 Make a world that has enough blocks to fly through without any gaps. The superflat template "Tunnelers' Dream" is recommended.

2 Run the command /gamemode 3

3 Fly through the blocks while pressing the "Run-Button"
(in Default Setting it's [Ctrl])

4 Try to look behind you while flying