▲你的怀疑是有道理的：这里的 some religious, some seasonal, and some for special people or events 不宜理解为省略句（不是承前省略），可理解为独立主格结构，可以视为省略了being，即：

Today’s festivals have many origins, some being religious, some being seasonal, and some being for special people or events.

但为方便理解，可以这样改写一下（破折号后面的内容为进一步的补充说明和解释）：

Today’s festivals have many origins—some are religious, some are seasonal, and some are for special people or events.

▲另外举几例“名词+形容词或介词短语”构成的独立主格结构：

He entered the room, his face pale. 他走进屋来脸色苍白。

He turned to her, his eyes ablaze with anger. 他转过身来面对着她，眼里燃烧着怒火。

He was waiting, his eyes on her back. 他在等着，眼睛望着她的背影。

The police officer had him up against the wall, both arms behind his back. 警官要他靠墙而立，双手放在背后。