Were are the bugs attacking me from?

In my factory, I inconveniently have 3 relatively large bug nests all really close to me. Only one of them was a problem, base number 2, but I have walled that one off.

All of a sudden though a get a message that parts of my base are getting destroyed. Turns out some bugs had managed to get to the red circle on my map. I have no clue how they got there though. My best guess is that they came from base 1 and came along the black line, but then why would they not attack my copper deposit? In any case, where did the bugs come from?

Your mining setup is quite small according to the radar image. As a result, the bugs follow in the direction of the strongest pollution. As they round the corner of the lake, that is both down. Once they get to the point where they would need to turn right to get to the mines, the pollution from the south is stronger still and the mines are out of the direct attack radius. Sp they follow south.

I don't have an exact answer for how they decided exactly what to attack, but:

1. The base on the left is within your pollution cloud. Spawners absorb pollution and use it as a resource to generate attack waves. You should plan to destroy every base that is exposed to pollution — this will greatly reduce the frequency of attacks. (You can also use efficiency modules to decrease the pollution output of buildings, but I imagine you're not to that tier of items yet.)

2. Regardless of pollution (I think), they will also send out groups seeking to build new bases in empty land, but if those exploration parties meet your buildings, they will attack instead.

You are very close to both of the bases on the left. You should remove them or expect to need to defend against them constantly.

Myself, I prefer keeping a wall and turrets completely encircling my base, rather than leaving any gaps at all, but some players build 'pillboxes' instead (groups of turrets). But in general, you should have some automated defense for all sides of your base — any open land, even without nearby bases, will sometimes get biters wandering across it, and even building new bases in it.