Its not one specific thing to be guarentee... did bunch of research cause just had this happen.
They are dying. Thats guarentee... but why is cause its sign of being pollinated, its sign of burning(foliar or nute), its a sign of it just dying off and new ones will grow.... those are the 3 answers i gather from ton of google searchs cause i was nervous an spent whole night. They are very vague and no way to know which so just keep heads up to not do thing which can make it happen again... like foliage feeding during late flower while lights on.