I had both urine and blood leaks (outside the catheter) during BMs. This is what I observed:
The muscle contractions involved in a bowel movement (both voluntary and involuntary) also put pressures on the bladder & urethra. These muscle movements "unseat" the inflatable spherical valve in the urethra holding the catheter in place and the urine leaks around it. The blood is from the newly sutured (sans prostrate) urethra that is healing around the catheter tube. The blood is probably caused by a combination of the muscle contractions slightly tearing the suturing - and the urine leaking past.
Once, after a somewhat constipated BM, bloody urine actually migrated back into my bladder and flowed into the bag. This upset my over-active imagination! As others here have suggested, eat foods that give you a soft BM for a few weeks.
My Physician's only concern about these "leaks" was that I not take aspirin nor other blood thinning agents until there was no sign of blood. Probably a good idea for at least six weeks for most (Ask you own doctor, please.) patients from any major surgery, anyways.
I'm in week 13 and all of you newly recovering patients, please be patient!
For weeks I thought I'd never be off pads again. I still leak a little, mostly after a long day at work - before I lay down for the evening. (I agree with the others, it seems to be fatigue setting in!) Some of us recover faster than others. I am believing all of recover in due time.
Because God first Loved us (cf. John 3:16), each of us is a temple for His Presence.