You have died from... 'Bad Luck'... What actually killed me?
For example, if you are killed by a secondary explosion -- if you are standing next to a fuel truck that blows up -- that is a Bad Luck death.
Similarly getting killed by unmanned moving vehicles (say the driver hopped out while the vehicle is in motion) or flying debris is also Bad Luck.
I'm pretty sure there's not any built in kill streak trigger. I've seen hackers ring up over 200 kills without a death.
If bad luck is not from something obvious (like an unmanned vehicle hitting you), I would say it's probably a bug.