It's not entirely certain but according to TheMaskedGorilla.com, industry insiders are claiming that there is an existing rift between the two artists. The alleged beef stems back to when The Weeknd signed a solo deal with Universal Republic back in September of 2012, and released Trilogy in November without crediting OVO at all, which is Drake's label. Drake sent out a tweet which was not directly aimed at The Weeknd but it hinted at a salty rift brewing between their friendship.
Just recently on February 23rd, the OVOFest sent out a tweet saying "Just OVO," which is another hint at a possible conflict between the two. The Weeknd in the past used the acronym "XO" on the OVO brand to make "OVOXO", however the "XO" has been taken off and apparently all of the OVOXO clothing has been removed from Drake's store.
Who knows what will the future will hold for Drake and The Weeknd working together again?