wriggle
蠢 centers on a slow, squirming movement, like insects or worms wriggling. It captures the idea of something alive and crawling in a clustered, restless way.
蠢 combines 春 (spring) with 䖵 (a doubled insect radical), suggesting the wriggling activity of insects emerging in spring. The historical development is consistent with this phono-semantic structure, where 春 may also contribute a sound element.
In spring (春), two insects (䖵) wake up and start wriggling around. Picture a mass of bugs squirming in the warm spring soil.
For シュン, imagine the wriggling insects are shunning the cold winter and emerging in spring: shun -> シュン, and they squirm out into the warmth.