any explosive weapon will sometimes leave duds, there are GMLRS, javelin and excalibur duds documented. ww1 era shells could be the worst because by some estimates up to some 20% failed, then fuzes were often brass so they didn’t corrode, but shell or bomb body were steel or cast iron so they did. when fuze gets almost set off then loses mechanical support it sometimes becomes more likely it’ll be initiated on its own
bombs are single use, there’s no need to pick it up now
why it was there in the first place? they probably had no idea, or if there was post-war cleanup it went undetected for some reason