The most common reasons for deployment include mid-air collisions, severe turbulence or rotor, and equipment failure.
Mid-air collisions can be avoided by not flying at crowded sites and/or being aware of your surroundings and other pilots. Know the right of way rules, but also make sure there isn’t a different pattern being used. Not every one obeys the rules. You need to determine if the other pilots are, and if not, avoid them or don’t fly.
Encountering turbulence that results in a loss of control, including loss of horizon reference, is a somewhat vague and generalized reason for throwing your reserve, as a result it is pilot and situation dependent. A pilot’s skill and experience is going to determine what they can and cannot control. As a pilot, you should be familiar with all the things that can happen to a paraglider and how to deal with them. Your altitude will be a major factor when dealing with a loss of control. Altitude equals time; the higher you are, the more time you have to deal with problems that may arise.
Flying into rotor will most likely result in a reserve deployment. If you end up in this situation, you probably didn’t follow one of the following paragliding commandments: keep a 2:1 glide to the top of the mountain or ‘if you’re going up, go out’ (in front). There are varying degrees of rotor; the strength of the wind and the height and shape of the mountain or ridge are factors. Your glider needs laminar airflow to generate lift. The airflow in an area of rotor is chaotic, worsening as you descend lower, and will make it difficult if not impossible for your glider to fly correctly. It is likely that you will experience deflations, and the glider will become increasingly difficult to manage. You do not want to have difficulty managing your glider as you near the ground, as this is the most dangerous time. Your best choice in this situation is to deploy your reserve. The parachute will handle the turbulence considerably better than the paraglider.
It is possible that you could have a malfunction of your paraglider; however this is very rare. With regular inspections and maintenance, your probability of having an in-air malfunction should be eliminated. Do not fly equipment that is damaged or not airworthy. Equipment malfunction is most often experienced by aerobatic pilots who put repeated stresses on their equipment. Anytime you are pushing the limits of the glider and your abilities, the chances of a reserve deployment are significantly increased. Deployments occur more often when pilots are flying in extremely unstable conditions, high performance gliders, and especially at competitions or performing aerobatic maneuvers. If you aspire to perform at this level, you should be aware that the probability of needing to deploy your reserve is in- creased.
The majority of problems that may lead to a reserve deployment can be solved with enough altitude, so most deployments are made at lower altitudes. In general, you will need a minimum of 300 feet to 500 feet to complete a reserve deployment sequence. Below 300 feet the deployment would have to go perfectly, and in most cases it is probably better to manage the glider rather than give up on it and not have either canopy work correctly.