I've never actually thought of this before, but you're right, that actually is a fairly prevalent occurrence in roleplay, with a pregnant (especially heavily-pregnant) character.
It kinda makes my blood boil, honestly, and could possibly be attributed to a lot of different things out in the real world, too (insensitivity to life, "unborn" life, and even whether it is a living soul, etc.).
But we're not touching that, for fears of this discussion going pro-life/pro-whatnot political, if you know what I mean.
That gets touchy.
My guess with this, though, is that it's another effect of the over-glorification of violence you see in many roleplays.
Let's look at one of the biggest and surprisingly most glorified character archetypes we see out in the public maps-- "The big, savage, cold, heinous heartless blah blah BRUTE." (Slightly exaggerated here, but you see my point.)
While some characters, especially testosterone-pumped males, may be pre-disposed to aggressive behavior, there are cases where roleplayers want to augment this fact by making their character... Well, more aggressive, or appear to be. They'll do this by making troubled-past bios like, "I killed my whole pack," or "I killed my father/brother/mate/etc.," and make them out to be ruthless characters.
Now, while these ruthless characters might like a big fight with a tough enemy.. But.
How fitting is it for this "big, savage, remorseless fighter" to hurt an 'enemy' in perhaps one of the most cruel and saddistic ways possible? Attacking a pregnant female, who is heavier and slower from carrying her precious baby, and kicking/attacking her until she has a miscarriage? Bereaving her, of her own cubs, kits or pups? Being forced to give birth to lifeless young, and not to mention the emotional trauma of the entire incident? What if it kills her, too? What of her mate?
That player's big brute would certainly look like a big, vicious beast now.
And so, the vicious, "standing up and fighting," "taking everything away from my enemy"-type character sometimes equals glorified in some roleplays, even if this "enemy" is a mother with helpless young. Not always, but it is a recurring theme.
I feel that--even if it isn't intentional--this is an underlying cause of why some players might see pregnant characters and their helpless unborn young as an easy target, and a "great way to boost their kill count," perhaps.