The thing is, now they have loads of cash in the bank (wasn't it $90M added recently?), they're worth suing. Nobody sues a pauper.
Not sure I quite agree with "The more attention you get there’s more potential for infringement.". It seems to me that the code either infringes one or more patents or it doesn't. Increased attention doesn't increase the potential for infringement; it increases the likelihood that an existing infringement will be discovered.
The other possibility is that he's maybe worried that in order to achieve faster development times, the Docker developers will, in an attempt to avoid reinventing the wheel, "accidently" steal someone else's hubcaps, as it were.