~$55 spread between Poloniex/BitFinex and Bitstamp/Coinbase. Normally, arbitrage would squash that kind of a spread. The only explanation I have is that there are withdrawal issues at Polo (we already know about BF issues) and/or deposit issues at Bitstamp and Coinbase.
2) the price of USDT tanked. it went down from $1 to $0.96 and that always increases the volume. and that is a 4% decline which is huge in something that is supposed to be worth a fixed $1 value.
Where did you see Tether tank to $0.96?