Why is my hardwood splitting apart and how do I fix it?
They say that wood never sleeps.

Seasonal fluctuations in relative humidity cause wood to expand and contract. Winter is traditionally the dry season, which causes wood to shrink. Similarly, summer is typically more humid, which causes expansion. 

This is absolutely positively not an infestation of any type.

Not sure why this wouldn't have happened last year, but local weather or changes to your humidifier may have been related.

Short version: don't worry about it.