A fork is usually caused by very low hash rate - whether or not it is more likely when solo mining, I would guess, is dependent on exactly how low that hash rate is. (sort of guessing here) 
I have got to learn more about blockchain construction, what's in it, how to interpret it. For instance, I just had a quick look through the block explorer in the wallet. Noticed that there's very erratic hashrates shown there from block to block. However, when I jump back to say block 200,000 or so I see the same thing. This scene and it's sparse tool-set has definitely brought out the old coder geek in me but let's just say that the last compiler I used was Borland C++ ... on my 386 (first one was Apple ][ Pascal!) Not really looking forwards to it but I'm also not looking forwards to being put out on the ice-floe this winter. S'true, they still do that up here.
The POS (Proof-of-stake) blocks have much lower difficulty than the POW (Proof-of-work) blocks. That is probably what you are seeing.