Grimm's law
The specifically-Germanic part of the change in which the first plosive became a fricative but not the following it seems to have been just an exception to Grimm's law. Under the normal operation of the law, voiceless plosives become fricatives in Germanic. However, if two plosives stood next to each other, the first became a fricative by Grimm's law, if it was not so already, but the second remained a plosive. That exception applied not only to series of two plosives but also to series of and a plosive, and the plosive was then preserved. In some cases, that gave alternations between two related forms, one with s-mobile and the other without, such as English "steer", Icelandic "stjór", Dutch "stier" (← *"steuraz" ← PIE *"steuros" with preserved ) vs. Limburgish "deur, duur", Old Norse "þjórr" (← *"þeuraz" ← PIE *"tauros" with regularly shifted ).