My recollection is that he found a grenade laying on the ground that one of his comrades had dropped several minutes earlier and he made the mistake of picking it up.
It's never been clearly determined whether the grenade was dropped by Cleland himself or one of the troops with him. In his own description of the event, even he didn't know for sure. In either case, it would have had to have been dropped some number of seconds rather than several minutes earlier for it to explode when it did.