Hello @BeyondForm, I am no authority so make your own decision about the following words.
When I was 16 I took mushrooms and had an experience with mahasamadhi. When they kicked in, my clothes felt so constraining that I removed most of them and went outside where, apparently, I assumed a meditative state in the standing position for what would eventually be several hours. The only movements were minor muscle adjustments such that my skeleton was perfectly balanced with minimal effort. The mosquitoes were terrible that day, but in this state they didn't touch me and I knew that they wouldn't. It was extremely peaceful and it was obvious that "everything is energy" (the extent of my vocabulary at the time). The state deepened and it felt like my awareness was a knot that I was slowly but deliberately untangling. Once the knot was undone there was only a string -- a binary option of life or death. It was clearly presented to me: this string is your awareness, you could end it and this body would be die but your awareness would continue beyond this realm. My childhood was extremely difficult and there was much suffering and it was clear this was a wonderful blessing. However, my friend came to mind, who was also taking mushrooms, and we were at his house while his parents were gone. It was clear that if I went through with it, he would have to live with the mess in 3d reality which would have a significant impact on his life. I also thought of my family. It was not an easy decision, but I decided to remain. There has been many hundreds of instances since that I have regretted this decision!
A decade or more later I would learn that this experience would lines up precisely with how I understand Raja Yoga (e.g. Patanjali's Yoga Sutras, Bhagavad Gita, Yogananda's writings, etc) and the concept of mahasamadhi.
So here's what this vessel says in response to your post: Mahasamadhi won't happen by acciden; if it happens by accident, it's not mahasamadhi. It is a choice, an action, that your consciousness will see available to it. It cannot happen unconsciously, because IT IS consciousness. It is not like swimming so far from shore that you fail to save enough energy to return home. The default state is for you to be "pulled" back to your body. Your exertion (to bring about the cessation of the fluctuations of the mind) is what is "pushing" you out of your body. If you push it far enough (quiet the mind enough) you will see the option of not continuing the body has become available.
Every few years I search around for perhaps another who has stood on the precipice of mahasamadhi in such a fashion. The closest story I've found is the one about Ram Dass' student (also appears in this thread), which brought me much joy. And regret :). This time around I found your post (5 years late, but better late than never).