Maybe it's to do with her recent brush with death.
I didnt even know she was sick.... had to google it, first article I found:
In a letter to fans on her website, Lavigne said that the song Head Above Water reflected her struggle, and that she wrote it as she lay incapacitated on her couch.
“I had accepted death and could feel my body shutting down. I felt like I was drowning. Like I was going under water and I just needed to come up for air. Like I was in a river being pulled in a current,” - Avril lavigne