I disagree, people buying and selling the thing as an investment is the prime common element.
Yes, that's a universal reason, the qualitative aspect - a common monetary denomination is the quantitative element. Two sides of the same coin.
I think this factor may be very important. People buy or sell based on various indicators. Profitable strategies are copied and applied again later, unprofitable strategies die out (although there is a noob/sucker factor). If enough people are using the same strategies that made them money on home depot stock when trading bitcoin, it could produce a self-fulfilling prophecy, thus explaining (at least some of) the similarity.