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I think the drop in congestion is a combination of multiple factors: 1) the market dying down lead to less deposit/withdrawal activity on exchanges, 2) wallets are improving their fee estimation algorithms, 3) exchanges are batching transactions and implementing Segwit, 4) it's probable that a significant spam attack has ended.
since there were 100k unconfirmed transactions and now it is only 2000ish i am going to say it was most probably a spam attack against bitcoin.
1) the "dying down" similar to big rises will lead to MORE deposit and withdrawals on exchanges. people want to sell and others want to buy because price is down. and it is only a small percentage rise. as i said it was 100k versus current 2k
2) still doesn't explain number of unconfirmed transactions
3+4) this two i agree with.