Did you use a dam of thick frosting around the edge to hold the filling in?
I am having this issue with my butter cream recipe lately, too. I am thinking it could be a heat/humidity thing. I tried adding my margarine while it was still kind of cold instead of room temp, and it helped. Do you think the shortening could be too soft?
if you don't already have sugar shacks dvd "perfecting the art of buttercream" you should invest in it. It will show you exactly how to get rid of the bulge and ice the cake perfectly. I have watched it many times and learn something new every time. as far as the icing consistency, when I know that I've followed the recipe and didn't make any measuring mistakes, I put in the fridge a few minutes to cool it down and stiffen it up a bit.
good luck
The very nature of icing is to be sweet
Humidity and elevations effect all icings. You may need to Cut down on the liquid a bit.
I have made the recipe using shortening but at different times of the yr. Again, humidity effects not only the butter or shortening it also effects the powdered sugar so it's hard to say exactly why it's soft. (Butter's consistency changes at different times of the yr.) Also elevation can effect ingredients.....like at h igh elevations (I live at 8,500 ft) it dries out everything super fast.
One must be able to adjust a recipe to suit your local and likes. I'm NOT suggesting major/total changes just some Minor adjustments sometimes will make a world of difference.
Thank you for your suggestions. I did turn the AC off and open up all the windows - the humidity might be up. The first time I made it (AC on, windows closed) it was perfect and the best tasting frosting ever! I did put it in the box to cool a bit. Another question, 1st time I used high ratio shortening this time crisco any difference?
I would imagine the change from hi ratio to Crisco would make a difference however I have always used Crisco or an off brand of shortening. I was having a problem originally with my frosting being sticky and I pm'd Sugarshack and she suggested I use less liquid. I did and it was WONDERFUL!!!!
I also highly recommend the SS dvd's they are fabulous. Even if your happy with your technique of smoothing your frosting you cannot imagine how much better it will be by following her technique.
THAT DVD is worth its weight in gold.............'.
I don't know it's pretty lightweight 
I'd say it's worth Sugarshack's weight in gold. All of her DVds are. 
Thank you for your suggestions. I did turn the AC off and open up all the windows - the humidity might be up. The first time I made it (AC on, windows closed) it was perfect and the best tasting frosting ever! I did put it in the box to cool a bit. Another question, 1st time I used high ratio shortening this time crisco any difference?
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