Ouch.
You shouldn't use boiling water when filling a hot water bottle, for exactly this reason.
Do you only fill 2/3 with water and squeeze the air out before putting the cap on too? If you don't the air expands giving the insides positive pressure.
Ever since I was 15 years old I've always filled hot water bottles in the same way, and have always let the air out. So it goes to show this one was faulty, you can see the split which isn't at the top
I would agree with you if I'd just filled it to the brim