Somewhere in your process something wasn't cleaned/sanitized. I, as well as many others here, harvest and repitch yeast as our MO with few to no issues.
You mentioned above pouring the yeast to pint mason jars, then decanting, then decanting again I believe? Not sure what all that was about but it's definitely not necessary. If your second/third decant took all the beer off the top of the yeast that is likely where your issue lies. It must have a beer layer on top to protect it from the environment above.
Keep it simple. Leave a little beer, 1/4-1/2 inch on the cake, swirl it up and pour to well sanitized jars, cover and refrigerate. I put the caps on, tighten then loosen 1/4 turn to allow depressurization in case the yeast kicks out a little CO2 after the transfer.
The beer will separate from the yeast and make a nice protective cover between the yeast and the air above. When you're ready to use it just swirl it up and use sanitary practices to transfer to the new fermenter or starter container.