This is a very old thread, but there are TONS of them across the web with zero solutions... so if someone else should search for this like I did, I want to save them the trouble.
I was also having the problem that my phone would jump into dock mode constantly, and forcefully removing the dock app did little to help. While the speakerphone issue is annoying, the WORST part was that it kept turning on my screen randomly, and it leaves the screen on. In response to others guessing what the issue is, having read up on it I can tell you what it is: car docks have slightly modified microUSB plugs that connect pins 4 and 5 in the EVO's microUSB port. Corrosion or dust or maybe even damage cause them to connect improperly, even when NOTHING is plugged in. I don't think it's a problem with cheap chargers or anything like that, it's just the nature of using a port every single day for months or years.
Anyway, the fix is to use an app called NoDock by NOTHIZE. You need to be rooted, but if you're not rooted, what the hell are you doing here?!?! It completely removed the issue for me, which is incredible. They do have a trial version that you can test for 7 days before you buy it, but they kind of have you over the barrel on this one: I tried a few other Play Store apps with no success.