When someone is taken into custody at a probation check-in and held pending investigation, it means the authorities have enough reason to believe a violation occurred but have not yet completed the process of confirming it. That is probable cause in practice, and it is enough to hold someone while the investigation plays out.
What triggered it could be a number of things. A failed drug test, a new arrest, surveillance information, something flagged in his background, or a tip that came in. Probation officers do not always reveal the specific reason at the moment of detention, and that uncertainty is one of the harder parts of the situation for family members on the outside.
The uncomfortable reality is that there is very little you can do to speed this up. The investigation will move on the agency's timeline, not yours. Calls to the probation office asking for updates are unlikely to produce much information while the process is active. An attorney is the one person who can actually make inquiries on his behalf and get meaningful answers, so if he does not have legal representation, that is the most productive step you can take right now.
In the meantime, keep communication open through whatever channels are available. If he has phone access from where he is being held, stay reachable. A letter through InmateAid lets him know you are there and aware of what is happening, which matters more than it might seem when someone is sitting in custody without answers.