Why is it that me or my friends start a squad together and start searching for a match we split up ?

Good question!

Ive noticed that too. I saw that it happened whenever people are connecting to the game and your friends connect after your team is already full.

this is done to balance out the sides if there is an unfair balance, not like in COD where you sit waiting for ages till the game brings in enough ppl, this uses ppl in the lobby to get on with the game.. once you have played through the first match you will then be united with your squad for any other match there after.. found this out by the same happening to our clan.