I've had more than a few folks express anger, frustration, etc. after I have turned on them... Most of the time they are in denial that any of their actions could have led to the attack. A lot of players could really improve their games with a better understanding of why they were betrayed.
1. Silence. If you don't speak to me over a period time, especially when asked about a set of orders, I am going to presume that you have turned unfriendly. If I am making a choices as to whom I should put my trust, your silence has just done you in.
2. Refusal to actually do anything. I certainly understand a little bit of caution. However, this is not a risk-free game. If you are so scared to do anything that you don't want to join an attack, you will be the target of the attack.
3. Not following through on promised orders. Even if you did not actually do something against me, if you do not follow through with a promised order, you will create mistrust. If you lack of follow-through causes our attack to stall, you really increase the chances that I turn on you.
4. Complaining that you don't trust me. Again, I understand that you don't want to leave yourself completely exposed and that you don't want to be stabbed. But, if you keep complaining to me about how little you trust me, I am going to think that your reliability is going to wane. I'm going to look for other options. Besides, it's Diplomacy. Nobody really should trust anybody. But, shut up and play.
5. Incompetence and NMR's. If you can't take advantage of a good situation or fail to submit orders, you are likely more dangerous as an ally than an enemy.
6. Giving short one-liner replies to orders I suggest. If you are not making other suggestions or taking the time to read them, I know you do not have enough invested in our alliance to be trusted. A good ally reads suggestions and looks for ways to improve orders.
7. General obnoxiousness. If you are rude, snarky or obnoxious, I won't want to keep exchanging press with you. It's unpleasant to read and I''ll be looking for a different ally.