Hey Munch,
I have played electric bass for over forty years... funk, soul, hard rock, etc. and I think I've only broken about three strings in all that time. I tend to agree with these other guys. There can only be two explanations... either something faulty with your bass... usually a bridge or nut problem, or you're pulling the strings too hard. Most of the slap players I know don't hit or pull the strings very hard at all. It just sounds like it. A lot of them even use light guage strings. One guy I know sets them way low to the fret board. Guys like Victor Wooten like to do that because they do a lot of hammer-on techniques and it seems to work better that way. He plays light and turns the amp way up! I use a medium guage string myself (D'Addario half rounds) because, unless I'm slapping, I play quite often with a pick. But I'm in the minority on that one! (McCartney, Carol Kaye, Sting, being a few others) I actually play harder than a lot of slappers, but the only time my technique has broken a string is when I was bending (stretching) a string. I don't think snapping or pulling it has ever broken one. You might check out Abraham Laboriel's video on right hand techniques. He shows just about all of them. And, of course, the guy who started it all, Larry Graham, has some good videos too.
Hope I said something of help to you.
Holy Crow
Senior Soul Slapper and general fool