I have seen this a few times with BP in some cases it urine + fecal matter which accumulate due to a blockage often causes by a huge piece of urate.
This can be massage out very gently, once the piece of urate causing the obstruction passes the rest will go out easily, again the trick is to massage gently not to cause any injury or rectal prolapse.
Sometime it's due to dehydration causing the urate to harden leading to a blockage sometimes it is due to a more serious digestive issue which rarely resolve itself.