I have worked as a International Volunteer at the TECC [Thailand Elephant Conservation Center] near Lampang since 2003 and have great respect for these animals AND their Mahout handlers [mostly 1 man/1 elephant for life]. I have seen several minor "incidents" during this time BUT nothing of this significance in my experience.
I am very surprised that the elephant threw off his mahout in such a manner BUT I am guessing that this particular mahout MAY have been ill-treating/abusing his charge over an extended period of time [which is also unusual in my experience with elephants BOTH African and Asian in both circuses and zoos]. That person usually comes to a very sticky end at the hands of the abused animal [ALSO witnessed by me on more than 1 occasion].
It is ALSO possible that the animal in question was in MUSTH [an hormone "imbalance" which can be alleviated by a change in the animal's diet discovered after many years of study by TECC staff].
IF this was the case then this animal should have been taken to the forest [for a rest period of several days or weeks] AND DEFINITELY NOT have been used for any tourist riding experience !!! DOH!!!
I have worked with elephants "on and off' from when I was 10 years old [I am now almost 72yo] AND have never experienced an injury of any kind. The elephants I have been fortunate to work with over many years are some of my best [and most loyal] friends ... SADLY many of them have now "left" us ; including most recently "TRICIA" from Perth Zoo in AU ...
The unfortunate couple have my best wishes for a speedy and long-term recovery ...