Actually the decline in pending tx set in right around the time when big exchanges like Coinbase were implementing Segwit as well as batch-withdrawals. Previously each withdrawal from each customer was done as an individual transaction, but now they are all batched together and go out as one big transaction.

People have thus speculated that last year's high transaction fee and backlog was basically the result of Coinbase (and a handful others) 'spamming' the network due to the fact they hadn't implemented batching and Segwit yet.