This post gives a good explanation of what happened https://steemit.com/sbd/@themarkymark/sbd-is-not-mooning. The short answer is that HitBtc, a small exchange that does NOT even deal in SBD or Steem, had SBD listed at $20. That artificially drove the price up on sites that include HitBtc in their averages, like CoinMarketCap.
I do, however, think you make a valid point on the liquidity of SBD and Steem. I have also been concerned of late with the large and seemingly growing spread between SBD and Steem. As a content creator who gets "paid" in SBD, not Steem, this spread is very troubling. SBD is meant to be on par with Steem and more liquid than Steem yet that seems to be just the opposite. So why are we not being "paid" in Steem?
RE: Steem Dollars price rise – but unable to sell at that price! And what's going on with Blocktrades?