What medium is the ultrasonic transducer in? Air Try calculating the time of flight for the signal to travel down the cable and back? Does it match the 1.4ms? , no cable, just 2 cm of wire, so no Ultrasound travels in all directions, the housing attempts to dampen it in all directions other than the face. It could be reverberating off the housing or a nearby object that isn't in front of it. Is the transducer focused or unfocused? If unfocused, again, it could be reflecting off something unexpected.
If this is an ultrasound reflection, it's quite a strong reflector and i would hope that you know about the reflector.
What is DG4053? A MUX?, YES, Are you using it in place of the usual Transmit/Receive Switch used in ultrasonics? YES, what "usual" Tx/Rx switch are you referring to? I'd be concerned using an DG4053 before an ADC as i imagine that you'll lose valuable information. I don't follow, why would I loose valuable information?
Why does the RX sit at -2.5V after being driven? I would have expected that it would return to 0V.