You think we might've pissed them off somehow?

[He remembers Bertie crashing into his room for safety, having offended an alien without meaning to.]

Easy enough to offend an alien if you don't know what the hell you're doing. If this is more about sending a warning than about killing spiders, they could end up targeting people here.

[And if it was only a warning, it would explain why there were no more traps around the fence- evidence of one explosion is enough to get the point across.]