When I was writing service at a Ford store (HOLY %^#*, that was 20 years ago...I'm getting old), we had an Expedition that almost rolled into traffic on a major highway with the driver and her 3 kids. She said the brakes worked great slowing form 50 MPH down to about 5 mph, then the pedal went to the floor and it slowly rolled ahead (luckily she threw it into reverse and then Park). Turns out the ABS was getting a speed reading and thought the brakes were locking up, so it bled the pressure off until they started turning again. Not good when you're at a stop sign... Not clear from your post whether this was high or low speed, but just another possibility. I second checking for codes, but see if you can read live data to check sensors as well.
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I have had numerous ABS systems trip with messed up sensor data. It is not uncommon for the open reluctor sensors to pick up metal filings from the brakes/road since they are magnetic and it freaks the sensor out. Clean them with brake clean and wipe them with a paper towel to get the filings off. This is for the open sensors with the exposed tone wheel/reluctor. Sensors embedded in a sealed wheel bearing require a scan tool to check them.
Often the wiring starts to go bad where it flexes at the control arm or ball joint and causes similar problems but those usually trip the ABS light since the sensor signal goes completly silent and not just erratic but anything is possible.
If it happens often and you can reproduce the problem then disconnect the ABS unit and see if it goes away. If so then it is a sensor/pump/controller problem. If not then start looking at the hydraulics/booster.
Hope it helps.
