There are four likely scenarios that may or may not apply to your individual circumstances:
1. Someone on your network has been doing something iffy and Google has detected it
2. A device on your network has been compromised, most likely by malware and that device is doing someing iffy (most commonly a SPAM bot)
3. Someone else using your ISP has fallen into either 1) or 2) and this has prompted Google to block the ISPs entire IP range.
4. Googles detection software has malfunctioned
For 3) and 4) you should contact your ISPs abuse team (often abuse@yourisp, but check their documentation) who can work with Google to resolve the issue, but I would also urge you to check your network and device security and consider, if you use Wi-Fi changing your WPS key.
Your confidence that using Kubutu means your machine cannot be compromised is I am afraid misplaced. There are a number of know exploits and malware targeting major Linux distributions(
example ). The lower rate of usage does make Linux less attractive as a platform for malware so it does still make 2) the least likely senario - but I would still advise making sure that your installation is fully patched and up to date and also that you check for any unrecognized daemons running in kernel mode - overconfidence breeds complacency.
