调用“根据OpenID列表群发”接口时，返回9500002错误：

接口：https://developers.weixin.qq.com/doc/offiaccount/Message_Management/Batch_Sends_and_Originality_Checks.html#_4%E3%80%81%E6%A0%B9%E6%8D%AEOpenID%E5%88%97%E8%A1%A8%E7%BE%A4%E5%8F%91%E3%80%90%E8%AE%A2%E9%98%85%E5%8F%B7%E4%B8%8D%E5%8F%AF%E7%94%A8%EF%BC%8C%E6%9C%8D%E5%8A%A1%E5%8F%B7%E8%AE%A4%E8%AF%81%E5%90%8E%E5%8F%AF%E7%94%A8%E3%80%91

报错：{"errorCode":"9500002","errorMsg":"masssend task too much, system busy, please try later rid: 662dfa08-0600096f-60fa982b"}

同一小时内，针对同一批人进行推送，测试推送不同的文章，有的推送成功，有的推送出现上述报错，已知推送人员未超过一月4次群发限制。

出现上述报错的原因是？