There is a giant weather system affecting the midwest and North east USA today. Many of DL's hubs, including NYC/MSP/DTW are all being affected by this weather, even ATL is being impacted indirectly due to planes/crews being disrupted elsewhere in the network. This means your plane is likely being impacted at some point in it's journey today, which will affect its ability to leave on time from YYZ tomorrow morning. It could be that it was scheduled to overnight in YYZ tonight, but won't make it there until the morning, due to delays in NYC. Or it could be getting in late tonight, and due to crew rest issues won't be ready to go out until after the 2.5 hour delay. Whatever it is, DL already knows it's going to be late, so is proactively posting the delay now.

With regards to being put on another airline, it is weather, so they aren't going to put you on another airline unless you are a major VIP or a very compelling reason. Your best bet if you need to get to NYC is to try and get on something tonight. Alternatively you can take a full refund of your DL ticket and apply those funds to fly on another airline (AC YYZ-LGA). A lot can change with this weather system still, so don't be surprised if it gets delayed further, or if the other airlines start posting delays later tonight, depending what happens with NYC market and their inbound flights/crews.

Edit: current JFK status, for reference (also YYZ is affected right now too....)


And current misery map from flight aware



With regards to your flight specifically (DL 5535 YYZ-JFK), its inbound flight JFK-YYZ (DL 5541) was originally scheduled to fly tonight 1521-1724, then stay the night in YYZ before making a return. It's been delayed until 1800-2003 (due to weather at JFK). So there is no plane available in YYZ to operate your flight until 8am, which is why you're delayed tomorrow morning.