Volume is calculated from daily buy and sell trades. Maybe there is just more of day traders that are buying crypto and selling it in some hours after they get a necessary profit? In such case you can take your 100$ and buy BTC and then sell it untill exchange fee won't eat all money. In this case those 100$ will cause thousands of trade volume.
I would say that with price rise more day traders appear who wants to use opportunity for profit. This still doesn't mean that the overal number of investors appeared. But I would say that investors are always present on the market but they are adjusting their activities according to the current situation on the market.
Some new investors will come if current small price rise becomes a trend and Bitcoin price gets up from the current dead point. But that is yet to see.