Thanks for many great comments. Some more of my thoughts follow:
it is remotely possible I could have hit the AE lock by mistake-- but this seems unlikely as in at least 1 case the images after the problem image were same subject matter and not problematic.
I wasn't using any manual settings-- at least not intentionally. As
NaClH2O suggests though, the knob could have been between settings though this too seems unlikely as were this the case one would expect the subsequent images to contain the same item in the exif, which it didn't.
Re: user error- Definitely true that I should have dialed down my iso- the lighting conditions had changed rapidly and I got caught up in shooting (it was a wedding) and didn't immediately adjust the ISO, so at least in the case of the image that was at 1//4000 of a second, that was a contributing factor.
Re: Flash- it wasn't on for any of the images and settings on flash compensation were normal for all settings. On ne of the images I uploaded and at least one from a year ago, it indicated that the flash was suppressed.
re:
cats_five question re: does this happen in full auto-- I can't tell from these images and can't necessarily find out now because a) it won't be sunny in beautiful Whistler BC til Monday probably (but we have great skiing at the moment), and b) the problem is very intermittent. However, it does appear from one of the images from a year ago that indeed it did happen in full auto (see below).
I too wondered if part of the problem was what Lowell suggested re: fast lenses not being able to go low enough in ISO settings.
I just checked one of the images that overexposed from a year ago and it was shot at 1/30; f=5.6; iso=100 focal length 300 on a pentax FA 100-300 4.7-5.8 lens. Camera was in auto exposure mode, and exif data suggests flash was compulsory but did not fire, patterned metering, auto white balance, and nothing else odd. Looking at this other image's exif data from a year ago makes me think the issue may be related to firmware or metering (because this lens was at low f stop, camera setting at a low iso). The fact that it did happen in full auto mode with varied lenses (some fast, some not), even in the absence of user error (iso was correctly set on some of the images from this year as well as last year's) makes me think that it is a firmware glitch or metering problem.
Of course I'll welcome any other thoughts. Will try to dig out the additional year old over-exposed images and review exif data on those as well to see if I can discern a pattern....