Canon's algos work in strange ways sometimes! :bsmilie:
Under such situations, going manual is the fastest, most convenient way.
Here's why:
Unless you're comfy fiddling with EC settings, shooting and checking LCD AND histograms, then re-adjusting till you get the desired exposure ... what's happening is that you're desperately trying to override the camera's programming, which is cascading sort of.
By going manual, you are keeping the two main exposure controls - shutter speed and aperture constant and under YOUR command, and if the scene does not present you with rapidly changing light*, going manual gives you the most control.
When my camera starts acting like that, and under certain challenging light conditions as mentioned earlier, I also switch over to manual WB and dial in the color temperature I feel is correct instead of letting the camera's Auto WB run loose over several frames. Practising this also forces you to learn how to 'see' and identify color temperatures. Of course, it helps if you understand the basics of light as well.
Alternatively, shoot a burst of more than 3 or 4 shots. This will allow the automatation to 'settle' into what it thinks is 'correct', which may not be the result you want anyway, and THEN you have to do the above-mentioned fiddle with EC bla bla bla ...
Note: * Looking at the horse's mane, I suspect it was a windy day. Was it? If it was, the leaves in the tress could have been moving enough (with the backlight hitting your metering sensors) to throw your readings, and thus explaining the shift in shutter speeds
as the leaves moved. That's one possibility anyways. I'm sure you could think of others.