Ball curves on straight throw
Solution 1:
When this happened to me, I took it as "accuracy decreases as distance increases". More difficult to catch Pokemon will stay back or move back if precautions aren't taken by the player (berries and better Pokeballs).
Anecdotally though, I think curving the ball purposely allows you to throw further because you can initially account for the curve and aim according.
Solution 2:
I'm not exactly sure what causes it, but it usually happens on harder to catch pokemon, especially if I start using berries and greatballs. However, I've had it happen with lower CP pokemon and using normal pokeballs, so it could easily be coincidence.
I'll throw the ball straight, and it won't be spinning before hand, however as soon as I throw it, it starts to sparkle and veer off as if I had thrown a curveball. I once had it happen the other way around where I spun it until it shook and all that and it threw it straight anyways, so my guess is just that the curveballs are finicky.
Not necessarily the greatest answer, sorry. From personal experience though it does tend to happen on the harder to catch after using a razz berry, but since that was just my own experiences, take that with a grain of salt.
Edit: And just going to mention since I forgot, I hold my phone and swipe with my index, never been able to do it one handed. :P I have an LG G4 with a thick case so usually I don't have to worry about my fingers touching the edges of the screen.
Solution 3:
I've had this happen to me far too many times. Typically I notice it when I'm trying to play with my thumb while holding the phone and releasing my hold tends to swing the ball off center. Although after getting frustrated from that I try to force curve it or start my throw from the middle of the screen which helps it stay straight.
The reason for this could be that hitting "far away" targets is meant to be harder so they are making it more difficult, or simply a bug or error in the game reading how you're throwing it.
Solution 4:
This happens to me constantly. I just run away and re enter and it is fixed. No harm done. Unless you used razz berry.