In most cases, a digital signal is much faster than an analog signal, meaning that it contains a higher frequency content. As you probably know by now, traces that are close to each other can have parasitic capacitance. As frequency increases, the capacitance between two traces begins to look like a short circuit and can result in something commonly known as cross-talk.
An analog signal can just as easily jump to an analog or digital signal via crosstalk; however, in reality, analog signals are most frequently composed of much lower frequencies than the digital signals.
Additionally, digital signals are simply more immune to noise. A 100mV offset on a digital signal is no big deal past the transition point. In a 10-bit A/D referenced at 3.3V, a 100mV can be 31 codes off!