One question... Did you get the suspension geometry re-aligned and set up after fitting the coil overs and new bottom arms?
It is always a matter of course to perform this even with standard coils and springs and ALWAYS with new bottom wishbone arms even if the steering track rod ends haven't been disturbed....
Everything can be fitted correctly, nipped up tight but without the corrected alignment you end up putting undue pressure on joints and bushes at angles that they are simply not designed to take.
Next issue is the bottom arm bush itself... There is three reasons why this can happen.
1. the bottom wishbone ball joint attached to the wheel hub assembly is not seated squarely and completely flush in the hole. It puts pressure on the other two bushes which will end up deforming their links and destroying the bushes.
2. Rear wishbone bush clamp is cross threaded when tightened up, giving the impression that all is nipped up yet still loose and out of shape... bear in mind that when a car is traveling at speed over imperfect roads, the wishbone is the first thing that can take the brunt before the coil spring and shocks can do their job of compression with 250kg+ of weight on it... Thus the crashing and knocking... One pot hole is all it takes to bend the clamp if not tight and true.
3. The coil-overs are far too hard and offering no initial give and rebound to the suspension components so the next item to give is naturally going to be the softest installed part, the bushes.