In this case, all of the characters were not even 平声 at all in Middle Chinese, but were all 入声 instead [Wiktionary's IPA]:
夹 (夾) /kˠɛp̚/
击 (擊) /kek̚/
All of the above started with voiceless unaspirated (obstruent) consonants, so are all 阴入 in Middle Chinese. For Standard Mandarin, these reflexes are notoriously unpredictable anyway. You may see that many (though not most) of the 多音字 of Standard Mandarin fall into this Middle Chinese tone category.
This, on the other started with a voiced plosive in Middle Chinese, hence 阳入. As this was an obstruent voiced plosive, in Standard Mandarin this is regularly to be merged into 阳平, so
dié would have been expected for 跌.
This leads to the question, how did speech in Beijing get distributed as such? Various hypotheses have been proposed:
- dialect mixing (much more important nowadays; there is an example of Jinan in Shandong being influenced by Standard Mandarin, increasing the number of 上声 because of its phonetic similarity to Beijing's 阴平);
- initial consonant (labial towards 去声, others toward one of the 平声);
- tone sandhi; some characters were more common in the middle of words, where they are more likely to be non-flat in contour (so more likely to become modern 阳平)
These are not conclusive; all that we can say is that the entering tone was particularly unstable, and variation in how it is pronounced is to be expected.