I've seen this happen a lot with the cheaper piping bags... The thin reusable piping bags are great for whipped icing or fairly thin buttercream, but they are horrible for anything very stiff or cold. It takes a lot more pressure to pipe something that is very stiff, and if the bag isn't thick enough it will start to tear. Canvas bags can also stretch too much and absorb oils, allowing buttercream / chocolate icings to seep through.
I suggest trying some thicker polyurethane piping bags. They can be a bit more expensive, but they will last 10x as long.
The bakery I used to work in used some cheap plastic-lined canvas bags, and I would dread work when it was very cold in the morning.. I would fill the cheap bags with rock-hard buttercream and pop holes in the seams almost every time. It got to where I had to mark which bags had holes so that i could tape them up before using it... 
Hope this helps 
wow thats really weird, I havent had anything like that happen to me, I only buy the wilton piping bags because thats what I can get around me, The cake decorating store is on the other side of the city and I dont have a car so I make due with what I have.. What brand are you buying?
My Ateco bags are good. For some reason they're more flexible, but stronger. I have lots of both brands, and I've noticed that.
Cakeastic, have you used those icing bullets? They're the best thing you can do. You don't even have to clean your bag afterwards. They stay new.
Ateco is a good brand that (if I'm not mistaken) I used at the bakery I used to work at. I've used the Wilton bags at a different bakery, and I used those mostly for whipped icing because they felt so plastic-y and thin. I do remember breaking at least one or two Wilton bags with BC icing... What I would get is the tip stretching out and sending the coupler flying into my cake followed shortly by a huge glob of icing.... I use the disposable bags at home (I don't do enough at home to justify buying resuable bags) so I'm no expert with all the different types; I just know what the places I've worked at have put me through 
Here is a link to some Ateco polyurethane bags. There are a little cheaper on this site than at the Ateco website (www.atecousa.net).
http://cooksdream.com/store/polyurethane-pastry-bags.html
I agree on the tips. They are darker and it looks like they would rust quickly. I have those too, but my full set is Wilton. Sometimes brands have good things that the other one doesn't.
I like my Ateco bags better than my Wiltons. I like my Wilton tips better than my Atecos.
On the other hand, people should probably be using more this trick so that everything lasts longer. You do waste a little plastic, but even cleanup is easier.
http://cakecentral.com/modules.php?name=Forums&file=viewtopic&t=588591&postdays=0&postorder=asc&&start=0
I also have these, very sturdy but pricey. I only bought two big ones of this kind.
http://www.pastrychef.com/SUPERFLEX-PASTRY-BAGS_p_1462.html
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