They're aware that someone has gone back and done something to the timeline. The big crime here isn't that someone mucked about with the timeline - that happens all the time on both sides of the equation (for and against change - sometimes to keep the status quo, you need to make a few changes to solidify it). The big crime is that the person who did this broke the central law of time travel - 'Never delve alone'.
Time-travellers always work in teams, with each member of the team having one (and only one) function, over which they have total control but outside of which they can do nothing. I'm still enumerating these. Moving through time and space is a function of *ALL* the team members acting in concert - which is what makes the betrayal so heinous. By going back and mucking about, the traitor has (secretly) learned about and mastered all of these functions, making him or her an incredibly dangerous person - one capable of (in theory) destroying time as we know it.