This has also been affecting me on a t2.micro EC2 instance with Ubuntu 15.10, most recently occurred with:
4.2.0-30-generic #36-Ubuntu SMP Fri Feb 26 00:58:07 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
For me it often triggers when installing updates (probably kernel ones in particular). Otherwise the server is not doing much so probably not much of a surprise it doesn't trigger on other occasions.
This has also been affecting me on a t2.micro EC2 instance with Ubuntu 15.10, most recently occurred with:
4.2.0-30-generic #36-Ubuntu SMP Fri Feb 26 00:58:07 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
For me it often triggers when installing updates (probably kernel ones in particular). Otherwise the server is not doing much so probably not much of a surprise it doesn't trigger on other occasions.