I have this problem since a long time ago (like two years),but i never gave importance because i always did push up or variations.And it is not like i have problems doing many reps or winning a match of armwrestling, it just starts shaking no matter if im tired or not.Why thah could be. It could be because of a muscle descompensacion?Because my triceps are less impressive than my biceps.
If you google this its caused by mainly fatigue and dehydration. Fatigue also includes overuse remember. Give it a rest for a few days, back off the intensity and start again. If problems persist, head to your local doctor and let them do their thing.