Overdriving speakers, or clipping?

Standard disclaimer applies: this was only a test (but the result has me curious).

I was driving a pair of Boston Acoustics A60 bookshelves (89db, 8ohm, 75w, 1" tweeter, 8" woofer - big for a a bookshelf) with a B&K ST1400II (125w), giving a new preamp the flight test. I turned it up about half crank and the sound started getting harsh, and cutting out. I quickly backed it off, and grabbed another pair of speakers:

I took a huge pair of Wharfedale towers (valdus 500, 93db, 8ohm, 200w, 3 x 8" and 28mm tweeter) out of the basement, and played these very loud. They went MUCH louder than the point at which the BAs were cutting out, with no problems. Sounded pretty good for cheap speakers, too.

I've heard amps clip when they run out of juice, but it's hard to imagine the amp ran out of juice pushing the efficient, smaller BA's. It must be something else causing this - like overloading the crossover or internal wiring of the speaker, and causing some other effect (or perhaps the amp IS clipping as a result of something happening within the BA when it got too much power).

Any explanations for this phenomenon? I've never overdriven a speaker before and produced audible dropouts - I've heard plenty of speakers distort and lose their cool, but this is a new one for me ; )