I agree with answes above, The cause for the cracking is electrical arcing between points (caused by a loose connection). Note: As electricity arcs it creates heat that can be a cause for electrical fires, so this should be checked out.
I would suggest that there's a possibility of a few things - in my experience though this would mean a loose connection at the back of the socket, or the socket itself is faulty (due to wear and tear). I recommend getting an electrician to check it over for you.
Projects that add value to your home - Phil Spencer
Looking to extend your home but don't know where to start? Property expert and Rated People ambassador, Phil Spencer, shares some thoughts on why loft conversions are such a popular way to create more living space and add value to your home.