Taken from another thread of yours:

In the 4th century BC, the Sennones (a Gallic people) sacked and plundered Etruria and Rome (wich was already independent)
This disaster forced them to rethink how their army should operate. Most of the inspiration, and the scutum shield, came from the Samnite tribes in southern Italy, though the Samnites fought with only one line of reserve plus skirmishers IIRC.
The fact that the Greek-style hoplite phalanxes kinda sucked in the rugged terrain of Samnite territory, and the Samnite proto-maniples didn't, doubtless also kind of had an influence given how much the Romans had to fight with those guys.