First of all you'd have to look at the spikes and see if they are the result of single transactions with a high transaction fee or if there were more transactions overall that day.
If you see individual transactions with a very high transaction fee then I'd say it's pretty likely to be a mistake by the sender. Here, for example, is the explanation for the spike on January 10th.
I've been doing raw tx's without a problem for weeks I guess the
exhaustion of working over 12 hours per day for the last week has
clearly taken its toll.
In general though, there is obviously no way to know if someone paid a very high transaction fee on purpose or not.