That looks like it could be brickwalling, where the Aerco's output levels were too high for the 744's input stage settings.
I agree that it looks like brickwalling, but your description isn't very good. First of all, the SD 7xx units can handle a +20 dBu signal before clipping (that's with the line-in set to 0dB gain). I'm not sure what the exact specs for the output signal of the aerco is, but I really doubt that it is that high. And that's not what brickwalling is. What
might have happened is that the signal from the mk21's was too hot for the aerco, so you were overloading the circuitry in the aerco. When this happens, the aerco reaches it's limit and then it won't output any hotter signal. So if you're just watching the digital meters on the 744, you won't see clipping, because the aerco can't output a signal high enough to cause a digital clip. That's brickwalling. It's hard to know for sure, because I don't think the aerco has any clip indication lights. The fix for this type of problem is to either lower the gain on the aerco, or if that is not possible, lower the level of the input signal before it gets to the aerco (so as not to overload the preamp).
edit to add:
how does it sound? If it doesn't sound too bad, then you weren't overloading the aerco preamp too much.
also, sometimes low batteries can cause pre-amps to overload at lower volumes. (that's what happens with the sonosax, I'm not sure how the aerco reacts to low battery power). but maybe the "fix" for this issue in the future is to make sure your batteries are always fresh.