How is it possible to open your glider without using a launch pad?
Its a part of the new game mode you mentioned, Soaring 50's. I have yet to play it myself, but IGN has posted an article about it. The first paragraph has the information you're looking for.
First up is Soaring 50's, which adds a new dimension to Fortnite's 50 vs. 50 mode by letting players glide all over the map. Instead of only being able to use gliders at the start of matches and by using special items, but Soaring 50's grants the ability to deploy gliders whenever a player jumps or falls from a significant height.