100 bonus xp ??? Where does it come from? [duplicate]

There are only four ways to earn 100Xp from what I can gather:

• The Excellent Throw! Bonus - awarded for Capturing a Pokemon when capture circle is smallest and the pokeball is thrown in that circle.
• Capturing a Pokemon (which is clearly stated)
• Catch 100th Pokemon of a Species - awarded for capturing 100 of a single species of Pokemon
• Defeating a Pokemon at a Gym - gained for each Pokemon defeated at enemy Gym.

I'm confident what you did is Catch the 100th Pokemon of a Species since this is kind of lengthy to state at the XP screenshot you've shown. It likely the developers just worded it as "Bonus." You've also stated that you just finished catching a Pokemon and since "Pokemon Caught" is clearly worded (and so is the Excellent throw bonus when performed), the last option remaining is that you caught your 100th pokemon of that species (it doesn't appear based off your question that you defeated a pokemon at a gym).