Are the huge transaction fees mostly mistakes or some other market indicator?
As mentioned, raw transactions are incredibly easy to screw up since you must explicitly pay yourself if you want change.

Here's a few scenarios besides unbalanced outputs which will produce huge transaction (Tx) fees:


forgetting to pay change
forgetting to convert to Satoshis in your code
misplacing the decimal (perhaps due to rounding errors)
swapping the fields using BCI or Bitcoin Core (this used to happen easily)
setting the Txfee variable in the core software on top of the built in Tx fee
a miner could spend time mining the current blockchain plus an additional non-broadcasted transaction with a high fee. If the miner succeeded, it would then broadcast and cause a spike in the graph. The miner wouldn't make any extra money, and it would be wasting time not contributing to a pool.