While you were working on the sprocket area, if you didn't put adhesive tape or ziptie arround the piston and cylinder, the piston worked its way out of the cylinder. Prepare everything to put back on and then compress the cylinder between your hands so the piston gets back in, then quickly put it on. Bolts will still compress it some so don't be afraid to tighten them.
When my dealer took it off the last time, he forgot to put the ziptie and the piston was almost all the way out. He didn't have enough force to compress it back in, so he used portable vice grips to compress it.
2017 DL1000 White
2014 DL1000 Red (sold)
2012 DL650 White (sold)