Macbook Pro freezes and reboots when held

My Macbook Pro (MacBookPro9,1 or MD103LL/A i7 2.3GHz, OSX 10.11.4 ) decides to freeze and reboot when lifted holding from the metal part under the trackpad and around it. It also does the same thing if I rest my elbows over the same places. What may be problem causing it? The device didn't ever fall from anywhere and so but suddenly decided to act strange. I have opened the back lid after this problem and cleaned any dust and/or debris but didn't find a noticeable amount either. The HDD was replaced with a SSD at Apple Store Amsterdam (Leidseplein) about 3 years ago and the problem has been occurring since about a year.