Why? Because of this new clip for the new TMNT show that was released:
And since Casey will be in the series too, I just know there will be people fighting over who April should be with. (I personally suspect most people will ship April/Don and then get pissed over the unavoidable April/Casey ship that'll become canon.)
Argh, why Nickelodeon, why do this? I know that the 2003 series also had Donny crushing on April, but it only started in season two and even then it was just a little bit. I can already see the enraged fangirls when April will ultimately break Donny's heart, calling her all kinds of names. Why can't she just be their friend?
And why am I so sure that April will end up with Casey? Because they always end up together in canon (well, not the old cartoon, but April didn't have a love interest in that one). I highly doubt they're going to change that and go for the interspecies romance instead.