Why did the hunnic invasion lead to the Germanic people migrating west but the rise of Rome didn’t cause a eastern migration

I’m guessing that by the rise of Rome, you mean the expansion of Rome north and east into Gaul, Germania, Illyricum, Dalmatia, Pannonia, Dacia, etc. 

I think part of the reason is that the Romans actually didn’t push that far northeast; the decision to stop at the Rhine and the Danube limited the “push” factor eastward. Likewise, the Roman policy of trade and tribute, establishing client kingdoms in Germania, Dacia, Sarmatia, etc. created a buffer zone that could absorb eastward migration.

And sadly, I think another part of it has to do with the Roman practice of imperialism, that the focus was on conquering and then Romanizing people where they were as opposed to driving them off the land, but even more so the focus was on enslaving people, which meant that people who might have become refugees to the east instead moved south into the Mediterranean slave markets. 

aspis vs scutum?

These are two forms of shields used by the ancient Romans at various times. The aspis is theshield we think of when we think of the Greek hoplite and (pre-Alexandrian) phalanx: a large round shield, covered in bronze, and largely supported by the shoulder and the Argive grip. Borrowing as they had much of their military technology from the Greek-influenced Etruscans, the early Romans used this kind of shield quite heavily. 

However, the aspis wasn’t that well-suited to the more flexible and less tightly-packed ranks of the maniple (a tactical formation that the Romans borrowed from their neighbors the Samnites) and so the Romans replaced the aspis with the iconic scutum. By contrast, the scutum was a curved rectangle, with iron edging and an iron boss in the center of the shield:

Without all of that heavy bronze plating, the scutum was much lighter, so that it could be wielded easily in one hand without the need for shoulder support, which made it easier to hold up the shield while throwing the pilum or stabbing with the gladius, as opposed to relying solely on spear thrusts. Moreover, the longer scutum did a better job at covering the legs and feet during combat, so that despite being lighter than the apsis, it actually provided more protection. And finally, the scutum’s design allowed for tactical innovations: the testudo formation, for example, wouldn’t really work with the aspis because it would be more exhausting to lock shields and you wouldn’t be able to huddle together closely enough, leaving gaps.

So yeah, scutum all the way.