A bit of a dramatic reaction to being darted! That being said, I'm sure I would react the same!
Although not what first springs to mind when seeing a dart, I was actually giving this hippo her contraception. Contraception is an interesting topic in wildlife medicine, and just like in humans, there are many different options. One of the main differences in the animal world is taking into account the "personality" and behaviour of each species. For example, orangutans often have contraceptive implants placed in their arm. Hippos and elephants are more challenging to be anaesthetised for the placement of an implant, so may be "vaccinated" either by dart or hand with a hormonal vaccine. Other species may be desexed surgically, but for some such as chimpanzees this may not be a viable option as it may change the social structure of the group. Then again, regularly darting chimpanzees is also not a good idea as they are a species that will remember the person who darted them and will hold a grudge for decades!
Luckily for me, I don't think hippos have the same memory or level of intellect 😉