Answer by taking a quote from the following article:

Migration Period art denotes the artwork of the Germanic peoples during the Migration period (ca. 300-900). It includes the Migration art of the Germanic tribes on the continent, as well the start of the Insular art or Hiberno-Saxon art of the Anglo-Saxon and Celtic fusion in the British Isles. It covers many different styles of art including the polychrome style and the animal style.

In the 3rd century the Roman Empire almost collapsed and its army was becoming increasingly Germanic in make-up, so that in the 4th century when Huns pushed German tribes westward, they spilled across the Empire's borders and began to settle there. The Visigoths settled in Italy and then Spain, in the north the Franks settled into Gaul and western Germany, and in the 5th century Scandinavians such as the Angles, Saxons and Jutes invaded Britain. By the close of the 6th century the Western Roman Empire was almost completely replaced with smaller less politically organized, but vigorous, Germanic kingdoms.  Although these kingdoms were never homogeneous, they shared certain common cultural features. They settled in their new lands and become farmers and fishermen. Archaeological evidence shows no tradition of monumental artwork, such as architecture or large sculpture in permanent materials, but a preference instead for "mobile" art for personal display, usually also with a practical function, such as weapons, horse harness, tools, and jewelry which fastened clothes. The surviving art of the Germanic peoples is almost entirely personal adornment, portable, and before conversion to Christianity was buried with its owner. Much art in organic materials has no doubt not survived.  Three styles dominate Germanic art. The polychrome style originated with the Goths who had settled in the Black Sea area. The animal style was found in Scandinavia, north Germany and England. Finally there was Insular art or the Hiberno-Saxon style, a brief but prosperous period after Christianization that saw the fusion of animal style, Celtic, Mediterranean and other motifs and techniques.

What happened in the 3rd century
In the 3rd century the Roman Empire almost collapsed and its army was becoming increasingly Germanic in make-up,