Prior to yesterdays 'event', the orderbook on bitrex (the only exchange trading ethercoin) looked like this:
0.00020 20 0.00030 15
0.00010 10 0.00035 20
0.00005 200 0.08 100
'Market cap' was 0.00030 BTC x 1000000 = 0.15 USD x 1,000,000 = $150,000
This figure is just the best price X coin supply and ignores the fact that the buy side of the book only contains 0.015 BTC.
Then, some fat fingered trader comes along and places a market buy order of value 0.0115 BTC, which eats into the sell side of the book, leaving it looking like this:
0.00020 20 0.08 100
Now, the 'market cap' becomes 0.08 BTC X 1000000 = 30 USD x 1,000,000 = $30,000,000
So, 'market cap' increased by 200 times, and yet there was only a buy order of roughly $4.
The mistake here is the market cap valuation using the best price. In markets that are this lacking in liquidity (like all cryptocurrency markets are), it is totally meaningless to calculate the market cap like this. To do it right, you need to clear the available supply against the buy side of the orderbook, aggregated for all market places.
note: these orderbook examples and trade sizes were simplified for illustration purposes.