When you send a very long text message, your device itself may decide to convert it into an MMS. In particular, this occurs when a text is longer than 3 messages. This depends on your device's operating system. You may try using specific applications which deactivate this limit.
The same principle applies when a text is sent to many recipients simultaneously. In this case, many devices will convert the message into an MMS. The tipping point is different for each device/operating system, but this is usually around 10 recipients.