Stress and breakage - Ralph MacKenzie

I'm looking for a little guidance on stress curves and their
interpretation, please.  I thought I was understanding the stress curves
and their implications, but having just had a rod break during test
casting, now I'm not so sure!

I recently bought a Pezon et Michel 7'2" LeMarvel from a friend.  I was
testing casting it with a 5/6 Wulff line and *really* enjoying the rod
and its power when suddenly, the tip section is hanging loose.  It had
snapped off pretty cleanly right at the top of the male ferrule.
(ARRRGH!)  The rod has a second tip, but now I am very concerned about
the durability of the cane under heavy loading during long casts, or if
I should inadvertently actually happen to catch a large fish of some
sort.  ;-)  (hey, you never know - these things do happen to some
people)

So, I measured it out expecting to see a stress rise where it broke, but
in fact, after getting the graph of rod stress, the rod broke at the
*lowest* point in the main body of the stress curve.  So, now I'm a bit
befuddled about how to interpret the curves, or the incident.  Could it
be that the cane was previously damaged?  The rod had hardly been
fished, if at all, so I don't think rot or anything like that was an
issue.  Or, like willows and oaks, high  stresses are where bending
occurs, and breaks happen when things don't bend - maybe?  I'm obviously
walking on eggshells when I take the other tip out now and would
appreciate a little help interpreting what happened, and what might or
might not happen with the other tip.

The taper, for those who may be interested (and it really does throw
nicely!):
0            68
5            87
10        104
15        119
20        136
25        150
30        172
35        189
40        204
45        209

50.25    218
55        233
60        249
65        257
70        270
75        297
77        298

Thanks in advance for any help you may be able to provide.

mac