People often forget that under severe stress or excitement behavioral communication often breaks down resulting in abnormal behaviors in captive animals. As an example, there have been anecdotal reports for a long time from people working with groups of toads where feeding would excite the males enough that they would initiate amplexus (I've seen this with both Wyoming toads and Southern Toads) attempts.
Anytime you deal with a species that defends reproductive resources and add a new animal to the group you have to expect aggressive behaviors as the animals work out home ranges. If the goal is to have them together then removing the animals and housing them together for awhile can help but you have to able to do this long enough that the territorial cues for the resident animal is lost.
Ed