Titanfall was originally planned to enter closed Beta on Valentine's day, which players could sign up to for PC or Xbox One beforehand to get a chance to play. Although the NDA was lifted for this event, it was still closed Beta for the first two days, and there was a fair amount of players who had yet to play, both on PC and the Xbox One.
However, since the event went pretty smoothly, Respawn wanted to stress test the servers as much as possible to make the launch of the title go by as perfectly as they can. To do this, Respawn decided to open up the Beta for full open access, meaning that all players could go into the game and access it, both on PC and Xbox One.
Vince Zampella seemed pretty happy to "break" the game, so it'll be interesting to see how the Beta plays out for the next few days.
The Titanfall Beta will remain open till the 19th February. After that point, players will have to wait for the full release later in March.
I managed to get my hands on Titanfall, and I feel like it's definitely got enough new content in it to make it feel like a fresh new shooter, but it doesn't take away too much from the modern day FPS to make it feel too unfamiliar for the average casual player. The Beta was limited for both maps and weapons, but this is likely to change on the full release.