Microsoft, Windows 7 Home Premium, Java - Jre7
In vanilla singleplayer, If you place a tnt-minecart on a normal rail, or powered rails ... (it did it with any rails) - and if it's pushed off by hand or by powered rails ... tnt-minecarts will detonate while falling in mid air.
I thought the minecart-tnt were only supposed to explode if they went over an activator rail. So I hope this helps.
MC-7786 Minecart with TNT explode when colliding