That's because you've installed a linter plugin, probably CSSLint or Sublime Linter. Disable or remove it and the borders at lines with errors will disappear.
Might even be my AndyEdits package (as they aren't errors - unless vendor specific-css is counted as an error or warning), and you've hit a shortcut-combination that displays edited regions.
"I'm here to save your life. But if I'm going to do that, I'll need total uninanonynymity." Me Myself & Irene.