Some of the stressors include, but not limited to:

Inconsistent lighting. (not on a timer)
Light leaks.
Overwatering. (root rot)
Cold at night, hot during day. (extreme temperature fluctuations)
Underfertilization, overferting.
Too many additives.
Unnecessary spraying.
Bad soil. (compacted, over-used, uncomposted, too fine of a consistency...)
Bad water conditions. (from the nuclear waste pond or a chlorine factory for instance)
ph fluctations.
Pulling-off fan leaves or overpruning.
Wrong light spectrums.
Insects.
High humidity. (mold)
Hormones/steroids or chemical treatments...

Could be one, could be more than one.