A number of things could explain the leak. The best course of action is to unstack the units, take the top panel off the washer and look for the leak. Start at the bottom and follow the water trail on up.
Things to inspect:
* Door boot: https://www.searspartsdirect.com/product/a78sr8c297-0026-110/id-w10290499
* Drain pump: https://www.searspartsdirect.com/product/377okqujmr-0026-110/id-w10130913
* Drain hose: https://www.searspartsdirect.com/product/4ajhfuuory-0026-110/id-w10272563
* Tub-to-Pump hose: https://www.searspartsdirect.com/product/2h867mds40-0026-110/id-8540012
* Outer tub gaskets: https://www.searspartsdirect.com/product/20btth5e7h-0026-110/id-8540090 and https://www.searspartsdirect.com/product/5wu4ujb4fw-0026-110/id-w10303340
Ultimately, there is no way to know until you follow the leak to its source. Could be a $5 fix or a $200 fix.