Well, this is something I know a bit about.  Van Beinum had started tenure
with the LAPO and suddenly died.  Solti was appointed but never took over.
Mrs. Chandler, the power of the orchestra in those years (late 1950s,
early 60s), appointed Mehta as assistant without consulting Solti, who went
into High Dudgeon.  And Mehta got the job.  He was in his 20s at the time.

The orchestra was directionless at the time, and had never been more than
middling.  Mehta, I think, did a very good job with them.  I have seen many
fine performances, but the classics never brought out his best.  I've seen
some very lackluster performances of Beethoven and Mozart, along with
inspired Mahler, Bruckner and Stravinsky.

Bill S