Lux Flyer

Pilots can refuse for multiple reasons that are almost never nefarious, as much as the person posting to twitter wants to try and make it out to be. If a pilots refusing a departure it's usually out of a safety interst or preventing further disservice to passengers . Frequently happens when they find a mechanical issue during their preflight checks (which are still ongoing even when boarding is happening), or in this case there appears (based on the delay notificaion) to have been a high probability concern given the ongoing ground/airspace congestion in EWR that they wouldn't actually get off in time to meet TLV curfew restrictions that could be imposed, in which case they would either be diverting enroute (yuck), or have a tarmac delay to just return to the gate when it becomes too late to take off (yuck). Both of which are a worse disservice then refusing the departure during the boarding process.

Edit to also add: might be poor form for them to have announced it as a refusal, since that is going to be interpreted differently to the general public than the airline/pilots/dispatch. General public hears refusal and takes it as "pilots just don't want to do it" or are being subordinate, whereas pilots use that term to send a clear message to dispatch that they don't feel it is appropriate to operate the flight as scheduled and will be hashing out the exact details with them. Frequently see it shown around when maintenance wants to defer something, but pilots don't feel it is safe to operate with that maintenance deferral so they "refuse" the aircraft.