When an individual attempts to sound smart by using two synonyms after each other.
'Hence why' does not equate to 'that's why'.
Hence Why ≠ That's Why
"... global warming is becoming an increasingly accepted theory; hence why we are seeing a rise in environmental groups... " - said by relatively intelligent people who have a loose grasp of grammar.
"I spent all day in the sun; hence why I've got a tan" - intellectually challenged, yet attempting to prove their IQ fits within the norm.