Sometimes batteries just fail no matter how much love you lavish on them.
The new AGM battery in my Triumph 900 Triple was installed January 2013. March 2014, it went Tango Uniform even though it was ridden frequently and kept on a Battery Tender Jr. Vendor replaced the battery, shipped free, under a prorated warranty for only $23.00. All good now.
Luck of the draw, I'm guessing. The Stabb AGM battery in my '81 R65 is going on 4+ years and is as strong, or stronger, than when first installed. I did remove the OEM VR and replace it with a solid-state unit from Mottorrad Elektrik.