Spiny Alien Thangs are a marvel of adaptive ability. With exoskeletons far to cumbersome to permit traditional reproductive techniques, Spiny Alien Thangs have learned to propagate by kissing.
This is not a little platonic kiss: It contains a fertilized egg which heads straight for the victim's stomach. Here it absorbs nutrients for days or weeks, like a parasite only... well, just like a parasite.
When it senses the most dramatic moment possible, it bursts forth from the stomach in a spectacular display of special effects. This explosion naturally kills the host.
But isn't it worth it for all the cool gore, especially if the host was merely an extra and not a highly-paid guest star?