It is not random. I assure you that professional designers spend a great deal of time iterating over the various aspects of their design. Designers think about the various ways they can convey the message through visual imagery. They write down different words relating to what they are trying to say through the design. They think about the interactions between the different words. They do research on similar successful design. They develop dozens of thumbnail sketches and rough comps of around 5-10 good ideas. These are whittled down, sometimes combined, sometimes entirely new stuff is added through team collaboration. Once you settle on a concept it's all about tweaking it until it's aesthetically where you need it to be while maintaining a clarity of message and brand identity. In this case, the message was meant to be hidden and subtle, while adhering to the somewhat playful, creative image that has defined Apple for years (look at past event decor). It was probably a joy for the designers to implement this idea knowing that they were hiding the icons in plain sight. And as a testament to their work, it went largely unnoticed online for nearly a daywhich is quite a long time in this day and age of instant communication.
I hope I have answered your question. While my answer might not be 100% accurate to the process used at Apple, this is an abbreviated explanation of process based on my experience as a designer. Though sometimes things just come to usthe final output is often far from our first "random" thought once it is iterated and developed into a fully-coherent concept. But is any though truly random? Our very thought patterns have been guided by our experience and interaction with the world around us since birth. But I'll leave that debate to the philosophers in the crowd.
