SMS messages are sent in what are called segments. A segment is a portion of an SMS message that contains 160 standard characters. When a message is long or complex, it's broken up into multiple segments for the text carrier to send.
When a message is split into segments, the third-party text service uses headers to tell the text carrier how to assemble the message in the proper order.
However, if a customer is in an area with a bad signal, the texts will be received jumbled / out of order since they're backlogged in the queue and then released all at once.