Why has my toe nail started to bruise after walking and hiking?

This is common: In most instances, it is because the toe nail is a little too long. It usually occurs on the longest toe, probably your second toe. If the nail is a little long, it hits the end of the shoe at "toe-off". This causes the nail to wiggle a little on the nail bed, the soft tissue under the nail. After many cycles, that can cause tissue damage & bleeding under the nail. The fix = trim your toenails.

Check your shoe fit: Inappropriate shoe size (too big or too small) can cause significant problems. If your affected toe is repeatedly striking the inside of the shoe (too small), bruising and pain result. Shoes that are too big can allow your foot to slide to and fro within the shoe resulting in repetitive microtrauma to the toe as it repeatedly strikes the shoe. Appropriate socks (snug & breathable) are also helpful.

Nail : This is called cumulative trauma. Your toe is striking your shoe and causing the trauma. There may be some foot or toe pathology so see your podiatrist for an evaluation.