He stated that he would not sign if a team drafted him after the 3rd round. Per the CBA, if he was drafted in 2015 and did not sign with his team he would be eligible to enter the 2016 NFL Draft, and the team that drafted him in 2015 would not be eligible to select him. Because he went undrafted he is not eligible for next year's draft, and thus he chose to sign with someone. It is also worth noting that because he was a UDFA, his contract is up a year earlier than if he was drafted, so if he performs well he will get a big contract a year sooner than if he was drafted in the later rounds.
All in all, it appears teams passed on him altogether due to some combination of lack of information about his situation and a fear of him possibly not signing and then losing the rights to him next year as well.