Six month view of the mempool...

A growing mempool, currently at 400K, means a certain percentage of transactions are expiring.   With a default of 14 days, that's 400K/14 = 28K per day or 5% of the practical throughput of 550K per day.

On Aug 10th, Bitcoin pushed an unusually large amount of transactions into the pool (120K).  

On Aug 20th and 21st, it repeated.

111K Transactions Dumped in 9 Hours
93K Transactions Dumped In 21 Hours

TLDR version - Bitcoin's bandwidth is saturated.   It's losing transactions and will lose more as traffic increases.