Reasons

Xander watched as the two cops just stopped. The duo were busy making out one second and then the next they were busy getting as far away from each other as possible. The two big macho looking cops seemed more than a little upset as they busied themselves with stuff in their cop car.

Taking that as his cue to leave Xander tucked the weird spellbook under his arm and huriedly ambled away into the night with his most unconcerned and innocent walk possible. He did not want to have to talk to the cops about what happened. Saying 'a wizard did it' probably wasnt going to cut it.

As he walked away he did ponder what had happened. His magical knowledge was lacking. But he knew enough to puzzle it out. Maybe. So first things first... state the facts, go over the bullet points.

●Red robed idiot robs book store.

●Cops interupt red robed idiot.

●Red robed idiot uses spell on cops.

●Cops start making out.

●Red robed idiot monologs.

●Intrepid adventurer Xander Harris uses baseball bat to defeat the red robed idiot.

●Intrepid adventurer Xander Harris cant reverse spell as the spellbook was written by morons and very little is labeled correctly.

●Spell runs out.

So.... Based purely on his beginners level understanding of magic... the reason the spell stopped was...

He knocked the red robed idiot out? No. The spell lasted for another five minutes after he brained the red robed idiot. The spell didnt need a conscious caster.

So the next possible reason is... Time? Short term spell. Lasting long enough for the red ribed idiot to escape. It was only ever going to last those ten minutes.

Possible...

However it could also be a power issue. Magic was like almost everything else, it needed a power source. The spell could have needed more power to continue but with the red robed idiot knocked out the spell just sputtered out without an infusion of power.

That was the probably reason. Or maybe it was both of the last two. Or whatever.

Xander looked at the spellbook and made a mental note that the spells inside were probably power hogs that didnt last long.