Old 8 bit decoder behavior
From your data sheet link:


  These devices feature high-performance, n-p-n output transistors designed for use as indicator/relay drivers or as open-collector logic-circuit drivers.


In other words, they are open-collector devices which can only sink current by driving low; a high output state would only be achieved by something else sourcing current.

In your keyboard case, that "something else" would probably be pull-up resistors on the input side of the matrix, which only get connected through when as keyswitch is closed.  If you look at the row drivers when no keys are closed, you see a confusingly identical situation where the outputs change between being "off" and "low" - which doesn't look any different.

To detect which output is being driven when the switches are all open, you could add your own weak pullups to source current.  But it's probably better that you just monitor the smaller number of address lines.

Presumably, what you are doing here overall is hosting the USB keyboard with an MCU having a USB host port, then monitoring the scanning of the target system, and planning to inject an appropriate keypress by driving an input line low at the right time.  Depending on how consistent the behavior of the target system is, it's possibly you might get away with only monitoring one select line and predicting the timing of the others from that; but if it is software scanned or otherwise inconsistent you probably need to monitor all four - or else change the keyboard handling software to accept input by some other means, like a serial port.