Although the actual cause for the accident can be contributed to 2 actions on your part, first, you miscalculated the amount of room available for you to change lanes, so don't just assume that just because you signaled that the other driver must allow you entry into the lane; and two, (and this one I could never figure out) you saw him approaching and instead of trying to get out of his way, you step on your brakes!!!
Now, while you could not be cited for this, your insurer has to include some sort of violation on its report to the DMV, and so here are two vehicle code sections to support your insurer's finding:
You were attempting a lane change and since you started it but never completed it, you could be considered at fault for an unsafe lane change VC 22107, and/or failing to signal for 100 feet before affecting movement VC 22108 or both:
22107. No person shall turn a vehicle from a direct course or move right or left upon a roadway until such movement can be made with reasonable safety and then only after the giving of an appropriate signal in the manner provided in this chapter in the event any other vehicle may be affected by the movement.
22108. Any signal of intention to turn right or left shall be given continuously during the last 100 feet traveled by the vehicle before turning.
However, there is room to argue that neither of the above code section can be cited as the proximate cause for a collision if/when there was only property damage and no injury or death, then you will most likely see the following code section being reported to the DMV as cause for the collision. Since you had not completely taken possession of the lane, and instead you were hanging over both the right lane and the left lane (commonly referred to as "lane straddling"), you would be in violation of:
21658. Whenever any roadway has been divided into two or more clearly marked lanes for traffic in one direction, the following rules apply:
(a) A vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until such movement can be made with reasonable safety.
(b) Official signs may be erected directing slow-moving traffic to use a designated lane or allocating specified lanes to traffic moving in the same direction, and drivers of vehicles shall obey the directions of the traffic device.