As you'll recall from the flashback, Jenna and Garrett were confronted by Ali on the night she went missing. Alison and Jenna (who was blind at the time) got into a fight, and Garrett ended up intervening and swinging Spencer's hockey stick at her. However, Alison ducked and was OK, but she held her finger to her lip, encouraging Garrett to keep quiet about what happened. Last we saw in the flashback, Jenna seemed to be under the impression Ali was dead.
Why would Ali tell Garrett to keep quiet about what happened? We have a couple of theories.
1. She was scared of Jenna in the moment. Although Ali seemed to be handling her fight with Jenna well and had an advantage (because she wasn't blind), maybe she just wanted Jenna to go away before something really bad happened between them at that moment. In this case, she probably planned to let Jenna know the truth soon enough.
2. She was up to something else. In the same vein, maybe she just wanted to get Garrett and Jenna out of there ASAP because she had her own plans. Though if that's the case, why confront them in the first place?
3. She was trying to run away. On the other hand, maybe Ali wanted Jenna to really think she was dead. If Jenna was involved with whoever was threatening Ali — or even if Ali just thought she was — maybe she decided it would be better if Jenna thought she was dead. Of course, that would only work if her plan was to go into hiding that night. But maybe that was her plan. If this is the case, did she succeed? Maybe she's still alive. Or maybe she was killed as she was trying to get herself to safety, like Maya (Bianca Lawson).
4. As a cruel prank. Ali looked pretty serious and shaken during that scene, but this is Ali we're talking about. It's possible she just thought it would be funny to trick Jenna for a bit.
Why do you think Ali did it? Let us know in the comments!