Why did my senderid change?
Changes of a set senderid usually is done to improve the delivery of the message. A lot of filters worldwide have rules to check certain senderids. Changing some/all senderids will allow to bypass some of the filters.
Some operators do not allow certain types of senderids (alpha or numeric) and block them straight away. By overwriting the senderid, these types of blockages can be avoided. Be sure to check all the details for the countries messages get sent to.
It can also be due the route used. A lot of routes (usually lower quality routes) are sent from simcards (=simfarms) with a fixed senderid.