How long are you in the brief for before a BM occurs? I ask this because after a few hours in the same diaper(messy or not) the elastics in my M4s start to break free from the standing leak guards, so this happens to me somewhat often. But I'm not sure how Tena slip Maxis and Id expert maxis hold up to constant movement and dampness.
If it's not the standing leak guards you're talking about then it might just be the plastic has bee stretched slightly since first applying the brief or your last few "movements" have just been more substantial than usual.
If this problem persists and you're afraid of getting caught and don't feel like delivering the "It's a medical issue" speech(I see your listed as IC), I would highly recommend picking up a pair of plastic pants or an equivalent type of diaper cover to keep any leakages, blowouts, or smelly smells a little better contained if you don't have one already.
Just a suggestion though, get a few. Wearing the same one over and over again will significantly reduce the life of the cover.
And sizing is everything, research and measure for appropriate fit around the thigh(most important fit on a cover).
You also said you are quite active, so I would recommend the GaryWear Active Brief covers, pretty comfortable, not really crinkly or loud, come in a few different colors, and the material is slightly less stuffy than just flat vinyl or pvc pants. They are a little on the pricey side though.
And if you don't feel like doing that, well.. might be time to switch up your choice of brief. Get something with better fit and more robust standing leak guards.
Well I hope this helps and you iron out this little issue you're having and the leakages are managed. Soiled pants and stains aren't fun >~<
If you have any further questions, feel free to message me and I'll try to get to answering them as soon as possible.