I checked and Indiana is 3 hours ahead of us in Arizona and the Indiana polls close at 6:00 pm Indiana time or 9 pm Eastern.
Trump doing his press conference at exactly when the polls close means he expects to win and have the primary called for Trump as soon as the primary closes.
I think that's good timing. Gives him a chance to watch how the press treats the results, whther any politicians make remarks, gives Cruz and Kasich an opportunity to "do the right thing" without stepping on each others toes, etc. Not saying any of that is planned, just that he is giving them space to do whatever they think is appropriate; and he'll wind up the evening's formal announcements.