It isn't trivially obvious, no, but the "k" sound is a fricative (EDIT: oops, nope, plosive - see response to this comment), i.e. the tongue rapidly moves close to the palate and then away, creating turbulence, and sounds with a sharp peak and amplitude. I.e. the waveform of a "k" or "kiki" sound looks more spiky. The opposite is the case for the "bouba" sound.

It is a bit mysterious why animals would all map e.g. sharp auditory waveforms to sharper / spiky lines (kiki figure), and then smoother sounds to smoother lineforms (bouba figure), but, reasonably, these senses evolved together and used far more shared underlying processing machinery than they do presently. After all, both light and sound reach us in waves and both carry energy.

Some primitive mush-blob perhaps just started processing all incoming energy waves together, differentiating e.g. smoother vs. more staccatto waves before making the distinction between e.g. sound vs. light. Or I dunno, but IMO it makes more sense when you consider evolution, and don't just look at processing now in isolation from the likely development of these senses.

The 'b' sound is a plosive too, by the way. It's voiced, vs the voiceless 'k'.

But the place of articulation kinda flips, from rear consonant/front vowel (kiki), to front consonant/rear vowel (bouba).

If you flipped the voicing, it'd be 'gigi' vs 'poupa', which to my ears at least, is a lot less clear cut.

k is a plosive, not fricative: there's a discontinuity in the flow of air, not a friction-sounding continuous airflow. that fits your point though, regardless.