I think Simba feels threatened, and you as his leader are supposed to address any dogs getting pushy with him,and since you didn't address this situation, he had too. it very important that he knows whose in charge, or he will take charge and that could end poorly.
I don't take Buddy to dog parks for that very reason, we take him to a local park, where we have doggie meet ups with other neighbours and their dogs. When any dog gets to push with Buddy I deal with that dog , if I don't Buddy looks at me first,(like "hey, are ya gonna deal with this") and if I don't ...he will!!
We are their leaders, they look up to us and we must lead them , but also defend them.
(I'm trying to look at this from a doggie point of view
)