TK's meltdown in "Goal Line", claiming that he's alone and he'll always be alone. Dani sits with him in the ruin of his penthouse and writes down her seat number for the playoff game, so he knows where she'll be.
TK took the fact that he was a father very casually at first, but was willing to reach to his son and the mother he had a fling with. His bosses happily inform him he's not the father, but his good cheer was immediately dashed when he heard this. He leaves the gift he was going to get him for Dani, and she pulls it out to find it was what the boy wanted; a jersey signed by his favorite athlete. Poor TK.
Dani and Matt breaking up. Unlike most other TV show break ups, this isn't because of some random dramatic event or other big plot event or something that otherwise causes them to question the other's love. This was something that was slowing in the making over the course of several episodes. The break up itself is quiet and emotional precisely because the two of them realize that Matt's desire for a baby versus Danni's lack thereof isn't something that's going to go away, no matter how much they love each other.
After finally going to rehab, TK runs into an old friend, and the two decide to bust out because TK hates being in rehab and believes that the Hawks won't be able to survive the playoffs without him. But when he shows up at the team's HQ, he's turned away by Nico, who tells him that the team wants nothing to do with someone who busts out of rehab. So he and his buddy go get drunk and then show up at Dani's house, but Dani refuses to see him. The drunken duo then head back to TK's place to drink and do more drugs. And then TK's buddy dies from an overdose...