I thought cross training was supposed to prevent injury?

So I went for a cross training bike ride today... Cycled to a friends parents for lunch- hadn't seen her for a few months and her parents for years! They're both into running so it was good chat, however, was quite disappointed that they were only 3 miles away, so not much of a ride. We took a different route home, which involved a cycle down north hill- top speed 25 mph. Even if a silly 4x4 driver cut me up after making such a point of over revving at the top of the hill to overtake us. It bothers me when drivers are that silly- don't they know that bike brakes aren't as strong as car brakes and even so, a car going into Another car is a lot less dangerous than a cyclist going into a car... I yelled abuse at him and shook my hand... With only the half a mile climb to go before we got home we were stopped at a traffic light and the road was uphill. I tried to stay in my toe clips, as its such a faff getting them on, especially uphil... Yippee the light turned green, I could peddle, so i did, but my front wheel was turned such that I couldn't peddle any further..... BANG, I fell onto the road! Very hard.... Grazed knee, grazed elbow, and totally bruised hand... Now sitting here with ice on everything hoping it doesn't affect my running this week!
But, it was quite funny! Both times I've fallen off my bike have been from completely stationary! How dense am I?!?